Added a _pb postfix to the generated Julia files.

The postfix will generate files as such:
    filename.proto ----> filename_pb.jl

This will make it easy to identify files generated by protoc so that they can be
added to a .gitignore file or something similar.
Merge pull request #19 from analyzere/_pb-filename-postfix

Added a _pb postfix to the protoc generated Julia files.