index NSNotFound when indexOfSubRipItemWithStartTime:

(NSUInteger)indexOfSubRipItemWithStartTime:  will return NSNotFound. So
add a SubRipItem at index 0 when init a SubRip, in this SubRipItem make
the textLine @" "(show nothing); so that when the index is NSNotFound,
it can set index as 0.

When use this class to synchronize the subtitle in an AVPlayer (use
AVFoundation), the methods
addPeriodicTimeObserverForInterval:queue:usingBlock: can show the
proper subtitle and also clear the subtitle.