Moved event namespace string to a variable.

Part of issue #73
Added event namespace to unnamespaced events.

Only on events that need to be removed during the plugin destroy(). Part
of issue #73.
Added element collection to track hooked elements.

This collection will let the plugin remember all of the elements it is
attached to so it can easily destroy() all instances. Part of issue #73.
Changed destroy() API method so argument is optional.

Now the destroy() method can be invoked without specifying the elements
to roll back. When it is called without an argument it will destroy all
PowerTip instances and the global events. Part of issue #73.
Added tooltip collection to track created elements.

This collection will let the plugin remember all of the elements it has
created so they can be easily removed by the destroy() API method. Part
of issue #73.
Changed destroy() API method to remove tip elements.

Part of issue #73.
Documented the change to the destroy() API method.

Part of issue #73.
Noted destroy() API method change in change log.

Part of issue #73.
Added no elements check to destroy() API method.

Part of issue #73.