protobuffers know what files they will generate for zips/jars.

Preliminary support for compressed archives as sources. Currently .zips, .jars, tars, tar.gzs, and tar.bz2s work for protobuffers. Code is fairly extensible, with generic functionality in zipped_sources such that not much code is needed in protobuf_gen (or other targets in the future).
That was a left-over comment from when I copied the structure from thrifttest, oops