Pex

* Bump setuptools & wheel version pinning.
* Unescape html in PageParser.href_match_to_url.
* Memoize calls to Crawler.crawl() for performance win in find-links
based resolution.

* Fix infinite recursion when `PEX_PYTHON` points at a symlink.
* Add `/etc/pexrc` to the list of pexrc locations to check.
* Improve error messaging for platform constrained Untranslateable
errors.

* Add support for `.pexrc` files for influencing the pex environment.
See the notes [here](
https://github.com/pex-tool/pex/blob/master/docs/buildingpex.rst#tailoring-pex-execution-at-build-time
).
* Bug fix: PEX_PROFILE_FILENAME and PEX_PROFILE_SORT were not
respected.
* Adds the `bdist_pex` command to setuptools.
* Bug fix: We did not normalize package names in `ResolvableSet`, so
it was possible to depend on `sphinx` and `Sphinx-1.4a0.tar.gz` and
get two versions build and included into the pex.
* Adds a pex-identifying User-Agent.

* Bug fix: Accommodate OSX `Python` python binaries. Previously the
OSX python distributions shipped with OSX, XCode and available via
https://www.python.org/downloads/ could fail to be detected using
the `PythonInterpreter` class. Fixes
* Bug fix: PEX_SCRIPT failed when the script was from a not-zip-safe
egg.
* Bug fix: `sys.exit` called without arguments would cause
`None` to be printed on stderr since pex 1.0.1.

* Bug fix: PEX-INFO values were overridden by environment `Variables`
with default values that were not explicitly set in the environment.
Fixes 135.
* Bug fix: Since
[69649c1](https://github.com/pex-tool/pex/commit/69649c1) we have
been un-patching the side effects of `sys.modules` after
`PEX.execute`. This takes all modules imported during the PEX
lifecycle and sets all their attributes to `None`. Unfortunately,
`sys.excepthook`, `atexit` and `__del__` may still try to operate
using these tainted modules, causing exceptions on interpreter
teardown. This reverts just the `sys` un-patching so that the
above-mentioned teardown hooks behave more predictably.

* 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.