Add Twisted protocol interoperability to ometa and parsley

This includes `ometa.protocol.ParserProtocol` with tests, `parsley.makeProtocol`, and an example of netstrings (with tests).

I also implemented a more comprehensive example at https://github.com/habnabit/txsocksx/blob/parsley/txsocksx/client.py and https://github.com/habnabit/txsocksx/blob/parsley/txsocksx/grammar.py . The API seems good from doing this implementation, but I'm certainly open to suggestions.