Added usemin settings for using revved images with JavaScript

Using regex pattern matching to identify image urls pointing to images/ and images with the given suffixes.
This allows for programmatically setting images without having to do extra work
Updated imagemin to a version that is not broken.

grunt-contrib-imagemin 0.6.0 crashed on png files on Windows systems (did not test other systems)
0.7.0 works fine both on windows and OS X