Protobuf encoding now happens in TrezorClient, and transports get encoded blobs
to (chunkify and) send. This is a better design because transports don't need
to know about protobuf.
It also lays groundwork for sending raw bytes feature (#116)
This commit also removes all vestiges of ProtocolV2 which was never used and
will probably need to be redesigned from the ground up anyway. The code is
still ready for protocol flexibility.
