pex 0.8.0

• 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.
  RB #785
• Refactor http handling in pex to allow for alternate http implementations. Adds support
  for requests,
  improving both performance and security. For more information, read the commit notes at
  91c7f32.
  RB #778
• 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.
  RB #1303
• Make pex --version correct.
  Issue #19
• 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.
  RB #1378
• Bug fix: Replace uses of os.unsetenv(...) with del os.environ[...]
  Pull Request #11
• 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.
  Issue #21
• Bug fix: The pex -s option was not correctly pulling in transitive dependencies.
  Issue #22
• Bug fix: Adds content method to HTTP contexts that does HTML content decoding, fixing
  an encoding issue only experienced when using Python 3.
  Issue #10