Use availabletasks plugin to create alternate task listing to grunt -h

This PR fixes #4 
![grunt-help-screenshot](https://cloud.githubusercontent.com/assets/283489/4639813/139f081e-5415-11e4-9662-4494a514eb0e.png)

This does not build upon #19 so needs to be revised to take that into account. I'm also a bit unsure on what, if any, standard we should have on where the "help" is configured per task.