Pex

* Fixes an issue where SourceTranslator would overwrite .egg/.whl
distributions already on disk. Instead, it should always check to see
if a copy already exists and reuse if there.

This ordinarily should not be a problem but the zipimporter caches
metadata by filename instead of stat/sha, so if the underlying
contents changed a runtime error would be thrown due to seemingly
corrupt zip file offsets.

* Adds `-i/--index` option to the pex tool.

* Adds `twitter.common.python.pex_bootstrap` `bootstrap_pex_env`
function in order to initialize a PEX environment from within a
python interpreter. (Patch contributed by kwlzn)
* Adds stdin=,stdout=,stderr= keyword parameters to the `PEX.run`
function. (Patch from benjy)

* The crawler now defaults to not follow links for security reasons.
(Before the default behavior was to implicitly `--follow-links` for
all requirements.)

* Improves scrubbing of site-packages from PEX environments.

=============

* Silences exceptions reported during interpreter teardown (the
exceptions resulting from incorrect atexit handler behavior)
introduced by 0.4.3
* Adds `__hash__` to `Link` so that Packages are hashed correctly in
`twitter.common.python.resolver` `resolve`