REFACTORING: SHKFormFieldSettings subclassed for SHKFormFieldOptionPickerSettings. They used to be in unsafe and inflexible dictionary

benefits:

- optionPicker settings are now strongly typed
- breaked retain cycle (memory leak) - sharer retained form, and form retained sharer in optionPickerOptions dictionary
- got rid of "horrible NSString hack" in Kippt
- much easier implementation of optionPicker