Sprite scaling for DOM with backgroundSize=cover

Currently it's not possible to scale sprites - they get clipped. Default behaviour should be to scale.
https://groups.google.com/forum/?fromgroups=#!topic/craftyjs/BFo8K3FgySc
Merge pull request #423 from cawoodm/patch-2

Sprite scaling for DOM with backgroundSize=cover
Added improved dirty rectangles algorithm.

Authors: Mark Logan and Ian Langworth, Artillery
Improvements to error messages.
Merge pull request #427 from statico/dirty-rectangles

Improved dirty rectangles algorithm
Point storage test to use crafty.js instead of the pre-built crafty-local.js
Modified Storage Api in Crafty to use Pouch
Fix issue with saving in wrong type
Add checks if document does not exist