Part of ZMQ C interface wrapped in classes

Created Context and Socket as wrappers, only implemented what was needed so
far.
Socket.send implemented, still getting message in pointer
Receive message via Socket class
Add MsgFrame to encapsulate the frames

Feels a bit overkill, did not yet find a better solution
Add Socket.recv_multipart
Make socket pointer private
Remove receive_one_message
Sending based on MsgFrame
Use Socket.send to send messages, hide MsgFrame from sending
Remove send_message function
Add constraint for Socket.send(T)()