Create a serialziation registry where Format[T] objects can be
registered for sbt-server to do dynamic serialziation/deserialization of
values.
Expose a mechanism to register serializers to the sbt build.
Add a test for custom serializers.

* Fix broken joda-time issues with making json Formats in macros.
* Add 'rawJson' for deserialized values if we had half of the
  serialization mechanism in place.
* Add a custom serializatoin test to UIInteraction.
Fixes from review.

* Remove exposure of global registry to APIs (while still using it for now)
* Fix name of classes so we aren't trying to execute a class that isn't an integration test
* Rename RegisterSerializer to Registered
Merge pull request #124 from sbt/wip/serializer-registry

Creates a mechanism whereby plugins/tools can register serialization.