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 updated this so that `collectstatic` is no longer exposed. Instead, I split `check` into `checkjs` and `checkpy` and the grunt watch task will now run `fab vagrant checkjs static:dev` if any JS file changes. What do you think?