Added kvc notifications

Added key value notifications when `frontViewController`, `rearViewController`, or `rightViewController` is set.

This way, another class (e.g. a menu table view) can observe observe changes to these properties and respond to such (e.g. changing which menu item is selected in its hierarchy).

Also, bumped patch pod spec version.

(Side note: I'm not seeing any unit tests in the main repo, or I'm looking in the wrong place? I can write unit tests for these if it will help to get this added to main repo. :)