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.

These changes probably should have been in 3b0fefb2 but
were overlooked.

(1) Move dependency on protobuf from grpcio to grpcio_test. While the
most-commonly-foreseen use case of grpcio makes use of protobuf,
technically protobuf is not strictly needed and there's no actual
in-code relationship between grpcio and protobuf.

(2) Loosen the dependency on protobuf from ==3.0.0a3 to >=3.0.0a3.

(3) Update all references to 0.10.0* to 0.11.0.

(4) Alphabetize the grpcio_test dependencies.

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.

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.

This allows for easier scripting and updates.