Add proto compiler gpb inttest

exercises rebar/gpb integration

The bulk of these tests are written by Luis Rascão, hence he is the
author of this commit. As the committer, I, have cherry-picked his two
commits 4c87bcd and ebb8182, from the feature/support_gpb_protobuf
branch in the git://github.com/lrascao/rebar repo, and have slightly
adapted it to fit this pluggable-proto-compilers-gpb branch.
Add inttest for default proto compiler (protobuffs)

This borrows heavily from the inttest for gpb, many thanks
to Luis Rascão for providing a most useful example.