track event for flush on background, manually add an event to the end of a batch when we flush
Lowered batch size to 40 and added a debug version number to events
Merge branch 'track-flush' of github.com:mixpanel/mixpanel-iphone into track-flush

Conflicts:
	HelloMixpanel/HelloMixpanel/AppDelegate.m
	Mixpanel/Mixpanel.m