Added public API to show the left and right utility buttons programmatically (related to #97).

New methods `showLeftUtilityButtonsAnimated:` and `showRightUtilityButtonsAnimated:` are counterparts to the existing `hideUtilityButtonsAnimated:`.

Added a toggle switch in the demo app that, when turned on, will exercise this functionality in response to row selection.

Also re-organized the code ViewController.m a little bit (some of the UITableView protocol methods were strewn around).