added unregistration of event handlers in the DidReceiveMemoryWarning

Based on the Xamarin memory management training sessions it is a good
idea to 'unregister' Event Handlers. In iOS 6 this should be done in
the DidReceiveMemoryWarning method and not the ViewDidUnload method.
added changed RefreshButton.Clicked to event handler to allow for unregistration
Deleted the Instruments trace files

I still have copies of these trace files if anybody wants them.
Changed RefreshButton to be private

Upon review with @gshackles the RefreshButton should become a private
propery on the Class.
Merge pull request #5 from cgleadall/event_cleanup

Event Handler unregistration