Implemented protobuf extends

This is one of 2 pull requests across protocol-buffer and resolve-protobuf-schema repositories that fixes imports of external protobuf files. now you can specify root_dir that defines base place from where all files are looked-up (as defined in protobuf specification: https://developers.google.com/protocol-buffers/docs/proto#other. 
You need to test it together with https://github.com/mafintosh/protocol-buffers/pull/13.