support jsmin.py from v8

v8 has its [own weird version of jsmin.py](http://v8.googlecode.com/svn/trunk/tools/jsmin.py) (maybe it's the original, or one of the originals, not sure...)

For better or worse, that's the jsmin Python module that is being shipped in Fedora right now so it would be nice if webassets could support it.