There may be some aspect of mortal sin in the way that context
management is now done in this test.

This makes grpc.early_adopter much more independent of RPC
Framework and cleaner at the cost of reexporting most of the
interfaces and writing several delegation classes.

This provides for now what should be a nicer interface for code
generation than that of the Face layer. In terms of abstraction
it's conceptually very similar so the two should probably be
merged as soon as is reasonable.

A top-level "grpc" package is added and RPC Framework
is made public.

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.