Sprite Generator fixes

This fixes a few things with the sprite generator and adds a couple of new features:

- Adds an optimisation pass for the sprite (file savings from ~24K -> ~9K)
- Fixes rounding errors causing sprites to leak into each other

![svg-error](https://f.cloud.github.com/assets/518943/1211196/d58157be-2605-11e3-8d26-86dcb37cdb45.png)
![svg-fixed](https://f.cloud.github.com/assets/518943/1211316/7cdfbdf6-2607-11e3-915a-113718abe425.png)

Sprite generation can be done through grunt now
**grunt compile:icons**

General image optimisation (both *public/* and *assets/* dirs)
**grunt imagemin:all**

*On Macs, this needs node >= v0.10.15*