added ability to specify asset compressor.

[skamansam]

These changes allow the specification of :js_compressor and :css_compressor to the sprockets method. This can allow for custom compressor classes or for specifying one of sprockets's built-in compressors, as a symbol. Updated README to reflect new changes. The default is still the same for the compressors.

[basex]

works great. thanks.

[wikimatze]

When I find the time, I will test and merge it, sorry for the delay.