One version per session

It's not valid for a frame in a SPDY session to have a different version than the rest of the session, so we should send the appropriate errors (either stream or session) when this condition occurs.

If you're interesting in seeing how this behaves, you can use Chrome with `--use-spdy=no-ssl` (which, due to a bug in Chrome, always uses `spdy/2`) and the non-ssl espdy-demo-server (available in my [draft3-no-ssl branch](https://github.com/bgentry/erlang-spdy/tree/draft3-no-ssl)).