Closes #6 Add per-endpoint serializer selection via Content-Type Header

Hi!

I have added a MessageSerializer Repository, which can be used to look up message serializers based on the Content-Type header.

I have also added tests that describe the process of selecting a serializer for serializing a message. However, I haven't added a test to cover deserializing. Although this is a general feature relevant to all transports, it is not relevant, and therefore not testable, on the Loopback transport. If you would want tests that at least cover one other transport (MSMQ), let me know and I can try to add it.

I have implemented this of the Master branch. If this is incorrect and you would like to correct this, please let me know.

Schmuik Raskin.