ISIS-323: starting to factor our json rendering...

... from the RO viewer.
ISIS-323: lots more refactoring of RO

* bumped to 2.0.0-SNAPSHOT
* renamed impl -> server
* moved the representations from 'server' to 'rendering' module
* representations depend only on guava's MediaType (might reverse this decision)
ISIS-323: back to using javax.ws.rs.core.MediaType

... reversed decision to use guava's equivaelent.
ISIS-323: RO publishing done, but no xactn stuff yet

* PublishingService
* EventSerializer
* PayloadFactory
* fix RO rendering so honours @NotPersistable
* extended RO rendering to suppress unimportant details when serializing event payload objects