Grunt decache

Added a Grunt task to clear the theme caches which is also carried out after compiling .less files when the default task (`grunt` or `grunt watch`) is invoked.

Run `npm install` in the theme root directory after pulling in the changes to install added node dependency.

I initially considered implementing this via a call to [moosh](https://github.com/tmuras/moosh). In the end I decided against based on the further requirements overhead this added (installing composer and moosh) to keep the tool easier to set up. Using the php executable seemed sensible as the developer must already have php installed to be able to run moodle locally.  Also `moosh clear-cache` clears all caches and not just the theme cache.

In order for decache to work successfully the user running the command needs to have write permission on the moodledata dir - presumably this will be the case anyway in most development environments.

**NOTE** you'll need to re-run `npm install` in the theme root to install the [grunt-exec](https://github.com/jharding/grunt-exec) plugin.