New layout for sprites, heavily "packed"

much more compact than "smart" when images' height and width are differently sized.

All credit goes to Jakes Gordon for the algorithm (and most of the code).

PackFitter algorithm description:
http://codeincomplete.com/posts/2011/5/7/bin_packing/

code adapted from:
https://github.com/jakesgordon/sprite-factory/blob/master/lib/sprite_factory/layout/packed.rb


See Issue #1422

## Comparison images

layout: packed
![mysprite-s792cf60370](https://f.cloud.github.com/assets/119071/1450235/ced757b8-4277-11e3-87ea-6866cfcdcb3e.png)

layout: smart
![mysprite-sf32dae54b5](https://f.cloud.github.com/assets/119071/1450231/b8b41570-4277-11e3-89bd-7ed695b1a51a.png)