-Rename namespace to grpc_health->grpc to match spec
-Proper use of NOT_FOUND status code
-Improve testing
-Add source distribution to artifact build

Now reasonable defaults are auto-detected by platform (and by specific
Python implementation).

We'll need to fix coverage testing in the future anyway (see #6894).

Before this change, running Python tests individually required
building a tox environment via the run_tests script and then specifying
long environment variables to filter out just the test we wanted to run
(and then we wouldn't be able to get the output on interrupt, nor would
we have an easy way of determining the PID of the process for debugger
attachment). Now invoking the build_python.sh script creates a workable
python virtual environment that includes all necessary libraries and
tests (s.t. running a single test is now possible by just knowing the
module name). This does not change existing supported means of running
tests (e.g. through run_tests.py).

An additional way of running individual tests has been introduced.
Following invocation of `./tools/run_tests/build_python.sh` (or
run_tests.py), one may invoke

  ./$VENV/bin/python -m $TEST_MODULE_NAME

and acquire a single running process that *is* the test process (rather
than a parent of the process). $VENV is the virtual environment name
specified to `build_python.sh` (defaults to `py27`) and
$TEST_MODULE_NAME is what it says on the tin.

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

Before it was possible for precompiled binaries of the Cython layer to
be downloaded during a test run if the metadata of the host machine's
gRPC Python project matched the metadata of a file in cloud storage. Now
we disallow this, and fix a bug where the relevant environment variable
was ineffectual.

The ideal fix would be to have a tox test command for running the
interop server and client.

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.

This is in preparation for the eventual refactoring of the run_tests.py
system.

Adds the initial reference implementation for health-checking in gRPC
Python as a separate project (but within the same grpc package to keep
namespaces consistent). Only installs the package to test the
build-proto-modules custom command introduced in the health-checking
project.

Also updates the version of grpcio_test. Now that grpcio and its tests
are in different project packages, the version numbers should be kept in
sync.

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

Conditionally runs the tests depending on the availability of Python
versions (because Travis is the worst).

Also set up environment-related details needed for a smooth Cython
experience: have the test script avoid rebuilding all dependencies if the
virtualenv directory already exists, have the PyPI distribution script
distribute the Cython generated C code rather than the .pyx files.