Add grunt watch task.

Run `grunt watch` to auto rebuild scripts when any JS file has changed.

Notes:

  * Run `grunt watch --dev` to build source maps.
  * The list of files to watch is cached when you run grunt so you'll need to rerun this command when adding files.
I still think it would be better if this was `tasks: ['check', 'shell:check_static']` and `command: 'fab vagrant static:dev'`

Then you could drop all the changes to the fab file.