organize factory menu as columns by category

We've had the `category` field in factory.json for a long time. Now we'll use it to organize the Factory plugin menu into columns.

![screen shot 2014-05-24 at 11 51 09 pm](https://cloud.githubusercontent.com/assets/12127/3076402/3f8c396c-e3d9-11e3-8458-e896cacd15a2.png)

Columns correspond to values `format` and `data`. Any other value will add the choice to a third, `other`, column.