Added Filter for serving gzipped files

Hey there,

This is a great little filter which improves the performance quite a lot. One thing which was missing, was that it doesn't also gzip all the gwt files. If a GWT project inherits from com.google.gwt.precompress.Precompress, then all its compiled files get their own .gz version, which contain the gzipped version of their output.

I've written a small filter which works great with those files. It checks if the incoming request has a .gz version, if so, it redirects to serve that version, and sets the correct encoding / content type, etc.

For example, if someone goes to example.com/Module/Module.nocache.js, they would be redirected to example.com/Module/Module.nocache.js.gz, with the proper headers set to serve it as a gzipped file. 

Hope this is useful. This filter works best if called **after** the GWTCacheControlFilter, since for .cache files, that filter would set the cache headers before the redirect is sent by this filter.