New implementation with no external dependencies

[iconara]

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).
• [ ] The Ruby MessagePack gem has gotten tests, make sure we pass those.
• [ ] Decide which versions of JRuby to support and make sure it runs on those.
• [ ] Run it through some stress tests to see that there's no remaining edge cases.
• [x] Decide on whether or not the extensions should be ported.
• [x] Decide whether or not to support the #to_msgpack convention from the Ruby version.
• [x] Decide whether or not to submit this as a PR to the Ruby MessagePack project instead of making a 2.0

[iconara]

Meh, forgot that the Ruby MessagePack gem had changed its API (not the top level #pack and #unpack, but the MessagePack::Unpacker is different and there's a MessagePack::Packer, too).

[iconara]

This is now a PR to msgpack-ruby, see msgpack/msgpack-ruby#33. It will have #to_msgpack (otherwise we can't merge), and extensions will not be ported (because msgpack-ruby does not support them).