Image optim

No longer must we manually optimize our images, now we can make grunt do it! This will take care of png's, jpgs', and gif's -- all automagically.

I added the directories `assets/images/src` and `assets/images/dist`. Put your images in `src` and run
```
grunt images
```

And all of your images will be optimized into the `dist` folder, so that the src folder will be the backup. Here's a screenshot:

![image](https://cloud.githubusercontent.com/assets/1478883/3096552/69ad9ac6-e5d5-11e3-8207-5a7f6835d7c0.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.