High res icons & logos for high density screens

Created a new mixin for the vendor prefixed density ratio media queries. Current this defaults to 2.0 which only targets Apple Retina devices (iPhone 4+, iPad 3+, Retina MBPs, etc) and some new Android devices (Galaxy S3, etc). Older Android devices have a pixel density ratio of 1.5 or 1.3, we can test and see how these fare.

I've also removed a whole heap of unused image assets in commits fd6669f57771e7580a24d0c047b8380b67678509 and 5b8280057e6c114cf438d9ba957265d492e10e16. So it is worth double checking nothing here is removed erroneously.