Serialize Request/ResponseMessage as Map in Kryo.

In this way, the Request/ResponseMessage does not need to be added as a custom class to Kryo.
Centralize tokens for serialization.
Remove ContentType from ResponseMessage.

ContentType is not needed because the format that the client sends will be the format that the response is formatted in.  It's not as though you can send JSON and get back Kryo.
Add a method to the Cluster Builder to add a custom serializer.
Added Serializers enum to make selecting a serializer nicer.
Remove the notion of an "accept" argument.

Use of "accept" doesn't make sense anymore.  The selection of the server-side serialization is detetermined by the format of the request.
The ToStringSerializer is no longer relevant.

Now defaults to plain old JSON.
Added an interal interface for supporting text-based websocket frames.

Implementers of custom serialization will not use text-based features so having those methods in the MessageSerializer wasn't so good.