Image optim

No longer must we manually run ImageOptima and ImageAlpha, now we can make grunt do it!

I added the directories `assets/images/png` and `assets/images/jpg`. Put your pngs and jpgs in their respective folders and run 
```
grunt images
```

And all of your pngs will be ImageAlpha'd, and both pngs and jpgs will be ImageOptimed (You can also have your jpgs use the .jpeg extension, but just put them in the jpg directory). Here's a screenshot:

![image](https://cloud.githubusercontent.com/assets/1478883/3046745/a46adc30-e12c-11e3-85c8-dbdd9110278c.png)

Obviously this has the potential to take a bit, so I didn't include it in the watch task. You manually have to call this, but that's all you have to do.