Create and use data model to represent choices in multiple-value settings items #3

Created model for selection items which contains a ```selectionTitle``` and ```selectionValue``` in ```RZMultiValueSelectionItem```
During parsing this we probably want to make sure these arrays are the same non-zero length. If not, I would throw an exception. It's OK to use `NSAssert` to do this, just make sure the comment is clear about what the problem is.

To elaborate, this is an error that would come up only during development (would not spontaneously happen) and would place the menu in an unusable state. Therefore it's totally acceptable to throw an exception here - it makes it very clear that something is wrong and provides an explanation of how to fix it. Much easier to notice than an error log statement and a menu in a broken state.