Move Format to Message (not subtypes) and clean up hackarounds

[havocp]

We want Message to always be serialized with a discriminator, and it will always be a sealed trait, so it makes sense to do the serialization at the Message level and make all the subtype serializers into private implementation details. This also removes the hacks we had (sendJsonFilter, etc.) to add the discriminator later in the process.

[havocp]

This is a net decrease in lines of code and it also privatizes a bunch of types that were public.

[jsuereth]

This looks ok to me. Might need to merge with @eed3si9n 's protocol tester thingy.

[havocp]

Now merged with master.

[havocp]

Now Travis-approved

[jsuereth]

LGTM