Add a modified patch from craigcitro@ to handle namespace sharing.

[silviulica]

ATTN: @haberman, @craigcitro
This is based on a patch from Craig Citro. I will need some hand holding to actually build the protobuf Py lib to do some manual tests. Did not manage to do a python setup.py build/test/install on a fresh clone.
I know that the init.py files will work because I am already doing some namespace sharing in google/ and google/cloud with other packages but still this needs to be manually tested for protobuf.
Thanks,
Silviu

[haberman]

This passed the tests, so I'm going to merge it for now even though I have not been able to verify that it solves the problems it was intended to solve.

[silviulica]

Any chance we can get this released (so we take dependencies). I can manually test if you give me instructions how to build the Python protobuf package or just point me to a tarball with the change.

[xfxyjwf]

@silviulica you can build the python package by running:

$ ./setup.py build sdist

In the python directory.

[silviulica]

When I do that I get this error:
google/protobuf/any.proto:31:10: Unrecognized syntax identifier "proto3". This parser only recognizes "proto2".

[xfxyjwf]

oh, you need to install the protocol compiler first:
https://github.com/google/protobuf/blob/master/src/README.md

The error message is from a pre-installed protocol compiler on your computer which seems to be an old version and can not understand proto3 syntax.