Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).

Notable Changes:
  -Convert all str types to byte types at cython layer (ascii encoding)
  -Use six for packages that have different names in Python2/Python3
  -By default, unit tests are compiled/run in Python2.7 and Python3.4
  -Ensure MACOSX_BUILD_TARGET is at least 10.7

The custom gRPC bdist command depends on numerous undocumented and
private behaviors of setuptools, wheel, distutils, etc. One such is the
ordering of generated distribution targets. We paper over this under the
assumption that the command will only be useful for gRPC devs, and
document with a command description a contractual obligation of users of
the command.

Adds doc-string to EggNameMixin, renames the custom install flag to make
it a bit clearer that the mechanism isn't necessarily limited to linux
(though certainly we'd like it to be; but that's a decision to be made
at the level of the build system and what is made available on GCS, not
here).

I have no idea how this mistake got through.

I'd been incorrectly using indirection against Cython's build_ext
module.

This reorganizes the Python code, scraps the current testing
infrastructure, and implements a simple test discovery and run script
based on the standard Python unittest library so we can trust that our
tests are running.

The old package directory handling was stale in grpcio's setup.py
command support module and docgen had a typo.

This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.