Fixed bug calculating NSTimeInterval to timeStamps that are in the past

Time difference is calculated from two uint64_t (unsigned) values. To get correct results when the first value is less than the second value, we have to cast the result to an int64_t (signed) value.
Merge pull request #71 from manide/master

Fixed bug calculating NSTimeInterval to timeStamps that are in the past
Merge branch 'master' of github.com:TheAmazingAudioEngine/TheAmazingAudioEngine