Add multi-up item grid widget.

Closes #214 
Closes #224 

I think that this has some styling considerations to take (especially around truncating the caption) but it should be good enough for the sprint 3 demo.

### Edit Form
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![screen shot 2014-02-20 at 8 39 03 pm](https://f.cloud.github.com/assets/96776/2226914/5098c722-9ab2-11e3-8d68-cc284fb79d72.png)

### Without Caption
---
![screen shot 2014-02-20 at 8 38 33 pm](https://f.cloud.github.com/assets/96776/2226915/5d195ee4-9ab2-11e3-9acd-1a2dbe18ca3b.png)

### With Caption
![screen shot 2014-02-20 at 8 38 47 pm](https://f.cloud.github.com/assets/96776/2226921/6a56be9e-9ab2-11e3-8681-615cbfe40d6b.png)