Remove MessagePack support from the library

After reviewing the possibilities with this Java API, we've decided that
JSON has better library support and we will not be using msgpack.
Implement JSON parsing and data binding as the default functionality

Removed the need for the raw messages to be directly accessible by users
of the library, as they can now just access JsonNode or do data binding
to their own type.
Update documentation to match new JSON features
Add missing @return to Javadoc