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.
Enable SDK users to localize the folder picker strings
Update cell alphas on setEnabled
Merge pull request #29 from box/ios7/folder-picker

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