Don't use GenServer labels in TransportSession

[edescourtis]

I am trying to add process instances of TransportSession to Swarm this library assumes that I can implement handle_call() with the :'$gen_call' label for a message it sends me. It seems to me that you should be using :gen.call with your own label inside the TransportSession module instead of using the GenServer label. This would avoid compatibility issues with other libraries. The user supplied module would then handle all :'$gen_call' and :'$gen_cast' requests.

[edescourtis]

At the very least it should forward all unknown requests to the user module. Assuming you want to keep labels as they are.

[savonarola]

Hello!

This is solved with #42 as far as I can see.