* Fixes #68 (nesquena/rabl issues tracker)
* Message Pack: http://www.msgpack.org/
* Defaults to using the Ruby msgpack gem: http://rubygems.org/gems/msgpack
* Adds Rabl configuration option `include_msgpack_root` which has same semantics as include_json_root.
* Adds Rabl configuration option `msgpack_engine` which has similar semantics as json_engine.
* Adds whole test suite that is essentially a clone of engine_test.rb, but with proper msgpack expected values.
Note that I converted the JSON expected values to their msgpack equivalent in this process.
* Updated README.md to reflect the msgpack changes.