Switch to grunt-contrib-less, add LESS source maps support

![screenshot 2013-11-06 13h 04m 08s](https://f.cloud.github.com/assets/115911/1485879/0b9edf48-471b-11e3-8f18-0539d7066b14.png)

to enable source maps, edit the `Gruntfile.js` - set the `sourceMap` to `true` and update the `sourceMapRootpath` based on your wordpress installation