JRuby support

_This is not complete, do not merge_

I've been working on integrating the in-progress v2.0 of [msgpack-jruby](https://github.com/iconara/msgpack-jruby) on top of msgpack-ruby. v2.0 does not depend on msgpack-java, like v1.x does, instead it contains its own Java implementation as a JRuby native extension.

I haven't had much time to work on this, but because of msgpack/msgpack-ruby#3 and f912169ecb22162d96c09e235f6f58f53fdc5bb6 I felt I needed to show that there is some progress. Please revert f912169ecb22162d96c09e235f6f58f53fdc5bb6 and the msgpack/jruby-merge branch, that version of msgpack-jruby is old, slower, and has unnecessary dependencies on bundled JAR files.

The work that needs to be done to get this to a mergeable point is to get `Unpacker` working (it's currently compatible with the old `Unpacker` from msgpack-ruby, but not the new one), and to fix some small issues with `Packer`. There are two classes called `Encoder` and `Decoder`, these should probably be merged into `Packer` and `Unpacker`.