enable visual feedback of currently selected cell

**[RSDFDatePickerDayCell]**
-added property to determine whether the user wants the cell to
highlight on selection.
-added selectedImageView & needed methods for easy customization of
selectedImageView’s property’s
-override `setSelected:` to accomplish highlighting
**[RSDFDatePickerMonthHeader]**
-added property to determine whether the user wants the header to
highlight.
-added selected property to save highlight state
**[RSDFDatePickerView]**
-added property to let the user choose if he wants to highlight the
current selection
-added property for the currently selected date (internal only for now)
-added `NSMapTable` to hold reference to the currently visible
supplementary views
-edited creation method of supplementary views and cells to add
selection state
-customized `collectionView:didSelectItemAtIndexPath:` to support
selection state handling