Replace Grunt with gulp

Replace Grunt builder with [gulp.js](http://gulpjs.com/). Tried to preserve the name of the tasks as much as I could, so they simply map from `grunt <taskName>` to `gulp <taskName>`. Please review. 

Fixes #10. 
Nope it's fine, makes sense to leave it in even if it's the default just to be sure. In the past I haven't had an issue either way.