Add a watch task to convince @djpaul that grunt is awesome :)

This pull request is super simple and should be all you need to get Grunt to compile your LESS files on the fly while you dev! :smile: 

All you should need to do is run `grunt watch` while you're working on your plugin and Grunt will look after the rest. I'm not a Mac person but I believe you Mac peeps normally use something like CodeKit to do this for you.

Hopefully this pull request addresses 50% of point number 3 of: https://github.com/paulgibbs/achievements/issues/116 :raised_hand: