iOS 6+ async bug fixed

The problem was, that on iOS 6 the asynchronous function requestAccessToEntityType was called. This function actually assigned self.eventStore after it was used the first time.
So you had to call this twice to get self.eventStore assigned and make use of it (tested on iOS 6.1.2 with a real device).
Merge pull request #14 from botweb/patch-1

iOS 6+ async bug fixed