Add `finishRecording` method with completion block (with backward compatibility)

iOS 6 SDK and higher: If writer responds to async method, call it, otherwise call synchronous method and completion handler.
iOS 5 SDK and lower: Call only synchronous method and completion handler. The rest should not be even compiled.
Fix case of nil completion handler, `AVAssetWriter` does not accept it.
Merge pull request #823 from iMartinKiss/pullrequest/finishRecording-iOS6

Add `finishRecording` method with completion block