Change base path for all assets

In my project, all my templates live in app/views folder, whereas all the assets live in app/public, similar to Rails structure. When grunt-usemin is building concat paths, it uses the view's/template's basedir, and for my project, this is not true.

For this reason, I added a new option to useminPrepare in order to set the lookup for all the assets. I also added a new unit test to verify this behavior (and specially, mimic my own project's usage).

Thanks!