Split out rhino version of lessc

This lets us have cleaner tests (since we don't need to include the test code in the rhino source). Also sets the stage for sharing lessc code between bin/lessc and rhino lessc (issue #14 in @SomMeri less-rhino repo) and provides a hook so other projects (like lesscss-java) can have js in between the rhino code and the rhino-lessc (or not use the rhino-lessc at all).

Note that the build.gradle had to move the source from javascript.source {} to combineJS so we could have control over the order that the files get included in the output.