JRuby support

This patch contains a JRuby compatible implementation of Snappy. It includes a JRuby extension and introduces a dependency on [snappy-jars](https://rubygems.org/gems/snappy-jars), which bundle Xerial's Snappy implementation for Java.

I created a separate `snappy-jruby.gemspec` for the JRuby version of the gem, since it can't have the `extensions` property set, includes another file (`lib/snappy_ext.jar`) and an extra dependency. To release a gem you now need to do one extra step: run compile (or better: run the tests) in JRuby so that the JAR file is created, then you can run `rake release` from MRI as usual (I've set up the `build` and `release` tasks so that they build and release both gems).