Smart layout now considers spacing, should fix #718

I basically added the spacing to every width and height computation, both for every image row in the smart layout fitter and in the fitter itself.

Since the output of this commit is a .png file with the sprite, I don't know how to build tests for this; however, since the changes are quite small and straightforward, and I have run some testing on different sets of images myself, I think it shouldn't break any of the existing behaviors.

Attached is a pair of sprites generated with smart layout, with 30px spacing and without the spacing rule in the .scss:

No spacing:
![registrazione-s05deac8370](https://f.cloud.github.com/assets/1030142/586934/fc3948ca-c959-11e2-87a0-4fe0d9b85b4b.png)

30px spacing:
![registrazione-s9bc14f8201](https://f.cloud.github.com/assets/1030142/586949/370c06fe-c95a-11e2-937e-e59dd7804809.png)