Pex

* Allow PEXBuilder to optionally copy files into the PEX environment
instead of hard-linking them.
* Allow PEXBuilder to optionally skip pre-compilation of .py files into
.pyc files.
* Bug fix: PEXBuilder did not respect the target interpreter when
compiling source to bytecode.
* Bug fix: Fix complex resolutions when using a cache.

The 1.0.0 release of pex introduces a few breaking changes: `pex -r` now
takes requirements.txt files instead of requirement specs, `pex -s` has
now been removed since source specs are accepted as arguments, and
`pex -p` has been removed in favor of its alias `pex -o`.

The pex *command line interface* now adheres to semver insofar as
backwards incompatible CLI changes will invoke a major version change.
Any backwards incompatible changes to the PEX environment variable
semantics will also result in a major version change. The pex *API*
adheres to semver insofar as backwards incompatible API changes will
invoke minor version changes.

For users of the PEX API, it is recommended to add minor version ranges,
e.g. `pex>=1.0,<1.1`. For users of the PEX CLI, major version ranges
such as `pex>=1,<2` should be sufficient.

* BREAKING CHANGE: Removes the `-s` option in favor of specifying
directories directly as arguments to the pex command line.
* BREAKING CHANGE: `pex -r` now takes requirements.txt filenames and
*not* requirement specs. Requirement specs are now passed as
arguments to the pex tool. Use `--` to escape command line arguments
passed to interpreters spawned by pex.
* Adds a number of flag aliases to be more compatible with pip command
lines: `--no-index`, `-f`, `--find-links`, `--index-url`,
`--no-use-wheel`. Removes `-p` in favor of `-o` exclusively.
* Adds `--python-shebang` option to the pex tool in order to set the
`!` shebang to an exact path.
* Adds support for `PEX_PYTHON` environment variable which will cause
the pex file to re-invoke itself using the interpreter specified,
e.g. `PEX_PYTHON=python3.4` or
`PEX_PYTHON=/exact/path/to/interpreter`.
* Adds support for `PEX_PATH` environment variable which allows
merging of PEX environments at runtime. This can be used to inject
plugins or entry_points or modules from one PEX into another
without explicitly building them together.
* Consolidates documentation of `PEX_` environment variables and adds
the `--help-variables` option to the pex client.
* Adds helper method to dump a package subdirectory onto disk from
within a zipped PEX file. This can be useful for applications that
know they're running within a PEX and would prefer some static
assets dumped to disk instead of running as an unzipped PEX file.
* Now supports extras for static URLs and installable directories.
* Adds `-m` and `--entry-point` alias to the existing `-e` option for
entry points in the pex tool to evoke the similarity to `python -m`.
* Adds console script support via `-c/--script/--console-script` and
`PEX_SCRIPT`. This allows you to reference the named entry point
instead of the exact `module:name` pair. Also supports scripts
defined in the `scripts` section of setup.py.
* Adds more debugging information when encountering unresolvable
requirements.
* Bug fix: `PEX_COVERAGE` and `PEX_PROFILE` did not function correctly
when SystemExit was raised.
* Bug fix: Fixes caching in the PEX tool since we don't cache the
source distributions of installable directories.

This is the last release before the 1.0.0 development branch is started.

* Change the setuptools range to `>=2.2,<16` by handling EntryPoint
changes as well as being flexible on whether `pkg_resources` is a
package or a module.
* Adds option groups to the pex tool to make the help output slightly
more readable.
* Bug fix: Make `pip install pex` work better by removing
`extras_requires` on the `console_script` entry point.
* New feature: Adds an interpreter cache to the `pex` tool. If the
user does not explicitly disable the wheel feature and attempts to
build a pex with wheels but does not have the wheel package
installed, pex will download it in order to make the feature work.

* Bug fix: Honor installed sys.excepthook in pex teardown.
* Bug fix: `UrllibContext` used `replace` as a keyword argument for
`bytes.decode` but this only works on Python 3.

* Bug fix: Fixup string formatting in pex/bin/pex.py to support Python
2.6

* Performance improvement: Speed up the best-case scenario of
dependency resolution.
* Bug fix: Change from `uuid4().get_hex()` to `uuid4().hex` to
maintain Python3 compatibility of pex.common.
* Bug fix: Actually cache the results of translation. Previously bdist
translations would be created in a temporary directory even if a
cache location was specified.
* Bug fix: Support all potential abi tag permutations when determining
platform compatibility.