Integrate JRuby implementation into the gem

[headius]

We managed to solve #169 by introducing a "stub" gem for the "java" platform, which contains only the .rb files and no extension. This allows JRuby to install the gem and include it in releases, but long-term it would be better if we can either:

• Integrate our implementation into this repository OR
• Release our implementation as a jruby-bigdecimal gem and depend on that gem in the bigdecimal-java gem

I have not started any of the work necessary to pull our BigDecimal implementation out of JRuby, which will take some clever trickery as we have code that expects it to be available all the time... when that happens we can move forward on this issue.