Implemented asPostData and tests for PrivateMessage.
Lets have messages know about where they should be posted. Adding test coverage around message types.
Moving away from static constructors for messages.
Moving some duplication into FlowdockMessage helpers. Added FlowdockMessage test. Pulling in mockito for test coverage.
Adding private notification option to plugin config.
Added private sender api token config option. Populated sendPrivateMessage method.