Fixed an issue where positioning the tabs was done unnecessarily often.

The tabBarFrameDidChange: method is called very often, but positioning the tabs and the background gradient is only needed if the notification is posted by the tabBarView, not its subviews.
Tab items are now animated when added to the bar if not the first one.
PLTabViewController now uses NSMapTables for tracking tab items, tracking areas, and view controllers.

This removes the need for assigning names to the tab items. Instead of three NSMutableDictionaries (each mapping name to one of tab item, view controller, and tracking area), the new approach uses two NSMapTables: mapping tab items to (1) view controller and (2) tracking area.

All methods that used to take a tab name now take a tab item itself. The activeTabName instance variable is now and activeTab.
Merge branch 'develop' of https://github.com/liasis/liasis into develop

Conflicts:
	Liasis/Liasis/TabView/PLTabViewController.m