JRuby compatibility

 Hey Sam, I would really enjoy a JRuby compatible version of the `yui-compressor` gem ...

Now I do not think invoking the compiler from the same process on JRuby is the way since compilers such as YUI and Closure tend to do a `System.exit` after their done ... but still there's a simple way of doing it "your way" - creating a separate `java -jar` process - without using `POpen4` which does a `fork` and thus it's having a hard-time running on JRuby ...

Tests pass on latest JRuby 1.6 as well as MRI 1.8.7 and 1.9.2 (and should work on Windows as well).