Extract out parsing proto file logic from ProtobufGen

1) The name 'type' shadows a builtin
2) Let's not use '%' style formatting, its going away in Python 3
3) Now that I see it this way, there is a shorthand that might make it look better.

classnames = set(['{name}OrBuilder'.format(name=t) for t in protobuf_parse.messages])
classnames = classnames.union(protobuf_parse.messages)