support for JrJackson

We're using JRuby and have settled on JrJackson as a faster JSON parser, we've included the support and all the tests pass. Since it seems that Jackson (the underlying Java-based JSON parser) doesn't support encoding of raw values (just Hashes or Arrays) we've included a method to fix this for rufus-json. We're not sure if this is actually a bug or a corner case in the JSON spec. If it's actually a bug we'll be more than happy to report it to the Jackson guys.