New implementation with no external dependencies

This removes the dependency on the MessagePack Java libraries in favour of a rewritten encoder and decoder that was made especially for JRuby.

The non-standard extensions that were in msgpack-jruby v1.x have not been implemented (automatic key symbolization and tagging of encodings).

There are a few things remaning before this is a candidate for a v2.0 release:

* [ ] Verify that this implements all of the new MessagePack spec, and does it correctly (for some definition of correct, the spec is what the spec is).
* [ ] Run it through some stress tests to see that there's no remaining edge cases.
* [ ] Decide on whether or not the extensions should be ported.
* [ ] Decide whether or not to support the `#to_msgpack` convention from the Ruby version.