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. 
Their protractor config doc says set it to `null` if you want to let selenium server find its own port. I see it works without it too by defaulting to 4444, but not sure if it's gonna pick another port if 4444 is taken. This is what I use in all my configs. I can remove it here, if you'd like.