Add protocol version negociation

These two changes fix some version issues with espdy.

The first changeset just fixed version handling to match what the SPDY2 spec says about which frames should care about version mismatch, and which shouldn't care.

The second changeset adds an additional option for spdy_version, when creating a session. 'negociate' tells the session to defer Zdef init until the client sends its first SETTINGS frame. At which point Zdef is initialized, and the spdy_version is selected. This allows espdy to accept both spdy2 and spdy3 clients.