Add recipe for google protobuf cpp implementation

[bakwc]

Protobuf with c++ implementation is much faster than native-python (10-60 times). Here is a recipe for building protobuf c++ extension. So instead of adding protobuf requirement, add protobuf_cpp one.
To ensure that you are using cpp implementation you can add folowing before importing protobuf:

import os
os.environ['PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION'] = 'cpp'

It will raise exception if cpp implementation is not available.

[inclement]

Looks good, thanks!