Convert custom folder picker cell to use standard UITableViewCell views instead of drawRect

These cells are just cells with the style UITableViewCellStyleSubtitle. The subview positioning
is retained.

This patch introduces a UI change: thumbs are displayed at a logical 32x32 instead of a logical
40x40. The thumbs returned by the API are either 32x32 or 64x64. On retina displays we use the
64x64 thumbs and they are displayed at native resolution.

Here's a look at how the folder picker now renders on the various iOSes

iOS 7:

![ios7](https://f.cloud.github.com/assets/860434/1007060/0bf18d28-0afe-11e3-85cc-4007ad0f41d4.png)

iOS 6:

![ios6](https://f.cloud.github.com/assets/860434/1007062/14b9f8dc-0afe-11e3-8d0d-35877b8f2b35.png)

iOS 5:

![ios5](https://f.cloud.github.com/assets/860434/1007063/1c338ccc-0afe-11e3-9b0f-835c4e94bfb0.png)