Max duration

Add a property accessor `maxDuration` to get the maximum duration of the current `_playerItem` that has been loaded. Returns `-1` if `CMTimeGetSeconds(_playerItem.duration)` is `NaN`. 

This will be useful in my own personal project to validate that the video duration is not greater than my own defined maximum (i.e. if the user gets slick and replaces the file on the device right before saving it to the database).