Optionally expose CSS settings

I'm queuing this up for a 7.x-2.0 release.

maxbraff requested the ability to optionally expose CSS settings to the node creator. The end result would be that a entity-type administrator would add the backgroundfield to an entity and specify that an entity creator could upload the image and also set the CSS value for a particular setting. The node creator would then be able to see specific setting widgets (e.g. position) but not others (e.g. repeat)

Implementation steps:
1. field_data table would need to have an extra column to save node specific css settings
2. instance would need to store exposed flags
3. instance exposure needs to be checked and applied to node form. node save would need to save to the field_data table