Python protobuf rules missing in bazel

[adorokhine]

Per #52, the "protobuf support" tracking issue is being split apart.

This issue is for the missing py_proto_library rules in bazel.

[adorokhine]

@cgrushko you asked to be cc'd on bazel issues for lacking protobuf support; here is one.

[cgrushko]

Thanks!
I can open-source our rules, but it'll take some effort.
Who wants to use it?

[aj-michael]

The Bazel Android tools would like to use it!

In particular, the tools that communicate with emulators that are in the process of being open-sourced need py_proto_library (but are currently planning on hacky workarounds instead).

[ghost]

Would love to have these as well. We're also using "hacky workarounds": pre-compiling our protos manually and checking in the resulting *_pb2.py files.

[kamalmarhubi]

This would be really useful. In particular, I'm trying to write rules for nanopb, and it has a Python protoc plugin, which needs descriptor_pb2 as well as its own options proto.

[tweeter0830]

I would also really love these.

[laurentlb]

Obsolete
(or if not, please file the bug in the corresponding repositories)

[brandjon]

Not really obsolete but now dupped by #3935 I think. Follow that bug for further developments.