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

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

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

The source code is moved from src/python to
src/python/src. A setup.py is added at
src/python. The build_python.sh and
run_python.sh scripts are updated to build
and run the Python tests by building a
package and installing it in the developer's
Python 2.7 virtual environment.