Use Imager.js for our responsive image solution

Uses a fork of BBC's Imager.js

https://github.com/oncletom/Imager.js/tree/feature-separation

When run, adds in the best-fit image to the container.
To work, we add the 'base' image url to the source, tell the lib what sizes are available, and let it construct the appropriate url. So we need profiles like https://github.com/guardian/frontend/compare/win-phone-bug?expand=1#diff-ab7bd6adb8d59f2b6ac4fe4c2ea369d9R33. Contentious?  - @commuterjoy 

Todo
 * update on screen resize
 * add lib to head, for quicker update?

cc: @kaelig, @phamann