Fix issues that arise due to accessing _player on a queue other than main

[isair]

For more information on why this is being done: #66 (comment)

Also renamed the method playerDidEnd as playerDidReachEnd.

[isair]

@brentvatne I need some clarification on a matter. Based on the fact that calling [_player pause] on the main queue has fixed the problem I mentioned in #66, I assumed react-native didn't call property setters just on the main thread. However, if that was the case, then there should have been problems with setSrc as well. There is no information in React Native's native UI component documentation, so I hope maybe you can shed some light on this.

[isair]

@brentvatne Merging this as-is without modifying the setSrc method as I haven't ran into a bug about it so far. Could this whole async business be a bug with react-native as it isn't mentioned in the docs?

[brentvatne]

Hey @isair sorry for the delay on getting back to you here - I'm actually not sure about whether or not setSrc would need to be called on the main thread. @johanneslumpe do you have any insight on this?