* added listener for timedMetadata event.
* added callback in RCTVideo for the timed metadata
* exposing onTimedMetadata to JS
* added forgotten method declaration
* returning array of string values
* added metadata type to the array
* added onMetadata method
* overridden onMetadata method on exoplayer2
* added format of return value from onMetadata
* added function reference in README file
- Use NSNull object in NSDictionary when type (or URI) missing from supplied
src object.
- In addition, defensively supply valid strings for URI and type from JS.
