shared secret support in obfs2
I coded (well, it was already coded but I made it functional) support for shared secrets in my shared_secret obfsproxy branch.
Some things that I don't know if I like or not:
- I currently set the shared secret per-connection, instead of setting the shared secret per-protocol on startup.
- I created a protocol_params_t struct that contains protocol parameters (eg, if we are the initiator, shared secret, etc.). It gets passed to proto_new() when we are creating a protocol object for every connection. Maybe in the future we will need to put more stuff in there, maybe not.