Bugfix and minor feature add

- Changed type of index on didTriggerLeftUtilityButtonWithIndex and didTriggerRightUtilityButtonWithIndex to NSUInteger to fix type conversion error. (Oddly this did not show up in your sample app, but did when I imported the code into my project so feel free to reject this part.)
- Exposed cellState as readonly property
- Added updateUtilityButtonsAnimated to allow programmatic setting of
  position.