Grunt tasks for easier development

I'm not sure what your workflow looks like, but I like being able to run the application in a web browser with live reloading. So I split the `build` task into 3:

  * `build` - Just builds the application to `/build/tmp`
  * `release` - Builds, uglifies and packages. The same as the old `build` task.
  * `dev` - Does the bare minimum build, sets up a basic development server and watches the `src` directory for changes.