Add grunt-asset-monitor task to Gruntfile

This adds a new [assetmonitor](https://github.com/guardian/grunt-asset-monitor) task to the Gruntfile which is ran as part of the ```dist-tc``` command on our continuous integration server.

The task analyses file sizes and logs the metrics back to Amazon CloudWatch where we can then create alerts and be warned when our static files go over certain sizes.

/cc @gklopper @rich-nguyen