Pex

* Performance improvement: Don't always write packages to disk if
they've already been cached. This can significantly speed up
launching PEX files with a large number of non-zip-safe
dependencies.

* Bug fix: Allow pex 0.8.x to parse pex files produced by earlier
versions of pex and twitter.common.python.
* Pin pex to setuptools prior to 9.x until we have a chance to make
changes related to PEP440 and the change of pkg_resources.py to a
package.

* Bug fix: Fix issue where it'd be possible to `os.path.getmtime` on
a remote `Link` object

* *API change*: Decouple translation from package iteration. This
removes the Obtainer construct entirely, which likely means if
you're using PEX as a library, you will need to change your code if
you were doing anything nontrivial. This adds a couple new options
to `resolve` but simplifies the story around how to cache packages.
* Refactor http handling in pex to allow for alternate http
implementations. Adds support for
[requests](https://github.com/kennethreitz/requests), improving both
performance and security. For more information, read the commit
notes at [91c7f32](
https://github.com/pex-tool/pex/commit/91c7f324085c18af714d35947b603a5f60aeb682
).
* Improvements to API documentation throughout.
* Renamed `Tracer` to `TraceLogger` to prevent nondeterministic isort
ordering.
* Refactor tox.ini to increase the number of environment combinations
and improve coverage.
* Adds HTTP retry support for the RequestsContext.
* Make pex `--version` correct.
* Bug fix: Fix over-aggressive `sys.modules` scrubbing for namespace
packages. Under certain circumstances, namespace packages in
site-packages could conflict with packages within a PEX, causing
them to fail importing.
* Bug fix: Replace uses of `os.unsetenv(...)` with
`del os.environ[...]`
* Bug fix: Scrub `sys.path` and `sys.modules` based upon both supplied
path and realpath of files and directories. Newer versions of
virtualenv on Linux symlink site-packages which caused those
packages to not be removed from `sys.path` correctly.
* Bug fix: The pex -s option was not correctly pulling in transitive
dependencies.
* Bug fix: Adds `content` method to HTTP contexts that does HTML
content decoding, fixing an encoding issue only experienced when
using Python 3.

* Rename `twitter.common.python` to `pex` and split out from the
[twitter/commons](http://github.com/twitter/commons) repo.

* Change the interpretation of `-i` (and of PyPIFetcher's pypi_base)
to match pip's `-i`. This is useful for compatibility with devpi.