Add initial node/transport_layer_test.py.

o Add tests for the callback/trigger logic
Add some comments to the TransportLayer callback functions.
Remove unused handle_raw_message().
Test the on_message function of the TransportLayer

o Add dependency to python 3rd party library 'mock' for use in unit testing
The TransportLayer.on_message() function is private to the p2p module. Add an underscore to make this more obvious.
Test TransportLayer.on_raw_message()

o Fix layer violation instantiating a CryptoPeerConnection
o s/init_peer/_init_peer/g
More clean up and tests

o Remove CryptoPeerConnection.on_message() it was a layer violation. Replace
  with XXX
o Use _ prefix on methods private to module
o Test TransportLayer.get_profile()
Merge pull request #103 from julianknutsen/clean-up-transport-layer

Clean up transport layer