SkypeExperience - adding support for sending messages to the server

The client now can send messages to the server but can not process responses from the server.

Bonus:
  - removing unneeded conversions
  - fixing a couple of tests that would occasionally break
TheyCalledMeBackAndLeftAMessage - adding message_received callback

The user can now set a callback which will be called each time the client receives a message from the server.