Feature/support gpb protocol buffers

Currently the way to build .proto files using rebar is through the erlang_protobuffs external projects. This project requires that all files be generated with the _pb suffix, this pull request aims not to replace but to introduce another protocol buffers project to rebar which contains many more features and capabilities, ideally it should gradually phase out the current way of compiling .proto. Feature set of gpb project is described in https://github.com/tomas-abrahamsson/gpb.
Capabilities for this integration of gpb/rebar are:
  - ability to define multiple .proto directories
  - ability to chose between recursive processing of directory or not
  - separate set of gpb options for each directory