Port pants to pex 0.8.x

SSIA.  The user facing parts of this change:
  - package resolution happens via requests now -- this is way more reliable
    and results in fewer Untranslateable exceptions
  - PEX_VERBOSE now controls all pex verbosity, including better messages around Untranslateable (yay!)
  - fixes a major pex regression with namespace packages introduced somewhere in 0.5.x

The pex 0.7.0 -> 0.8.0 changelog:

* *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](https://rbcommons.com/s/twitter/r/785/)
* 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/pantsbuild/pex/commit/91c7f324085c18af714d35947b603a5f60aeb682).
  [RB #778](https://rbcommons.com/s/twitter/r/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](https://rbcommons.com/s/twitter/r/1303/)
* Make pex --version correct.
  [Issue #19](https://github.com/pantsbuild/pex/issues/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](https://rbcommons.com/s/twitter/r/1378/)
* Bug fix: Replace uses of ``os.unsetenv(...)`` with ``del os.environ[...]``
  [Pull Request #11](https://github.com/pantsbuild/pex/pull/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](https://github.com/pantsbuild/pex/issues/21)
* Bug fix: The pex -s option was not correctly pulling in transitive dependencies.
  [Issue #22](https://github.com/pantsbuild/pex/issues/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](https://github.com/pantsbuild/pex/issues/10)

Testing Done:
build-support/bin/ci.sh

Reviewed at https://rbcommons.com/s/twitter/r/1421/