Creates a mechanism whereby plugins/tools can register serialization.

[jsuereth]

Allow plugins/tools to register serialization handlers on both sides of the Json protocol.

• DynamicSerializer registry of Manifest -> Serializer.
• API hooks in UIPlugin for sbt plugin authors to register serializers of their data
• Expose raw JSON values if serialization is only registered on the server
• Memoize "autogenerated" serializers (Attributed[], Seq[], raw types).
• Add an API to use DynamicSerializers when firing events rather than forcing the implicit Format (with caveat that this could be more dangerous).

Notes: This is still missing some way of preventing memory leaks on registered serializers, or evicting registered handlers on a classloader-death basis.

Review by @havocp cc> @skyluc

[jsuereth]

Also in terms of how to register serializers, I'm trying to figure that out practically...

I do want reusable "register serializer" code.... it's the only practial way to go.

[havocp]

Travis failure may be a real problem?

[jsuereth]

@havocp It's odd, it works locally for me. Do you mind trying it out?

[havocp]

Tests pass locally for me too.

Is this ready to merge then? Your fixes LGTM

[jsuereth]

Seems fine to me, but I may test on windows quick