Grunt Contrib Watch

This upgrades us away from grunt-nospawn and to grunt-contrib-watch which seems to be working properly as of 0.6.0. 

I've set `spawn: false` in the options, but purposely left out livereload as it is configurable in [lineman-livereload](https://github.com/linemanjs/lineman-livereload) which is only usable once this is merged.

If anyone has bandwidth to test, you can clone https://github.com/davemo/lineman-livereload-testing, npm install and `lineman run`. If you have the Chrome live reload extension, enable it once lineman is running and you'll get live reloaded css/js changes.

Would love some testing to see if this is mergeworthy in any larger Lineman projects you guys have available. /cc @searls @jasonkarns @jayharris