Add pause stop and finished event notification

[cymen]

This PR adds support for:

• pause
• stop
• unpause (just call play without a filename)
• event notification when playback has finished

For the event notification, it made sense to register the callback when creating the player which means it made sense to have the player be a class we create an instance of. It doesn't have to be that way but it seemed straight forward. So I did that.

[cymen]

(it includes PR #2)

[ghost]

@cymen what are your thoughts on setting the play, pause and stop methods static so there's no need to call new on the class returned by AudioPlayer.js? Here's an example.

[cymen]

@JHabdas That might make more sense -- I don't recall why I thought it should be an instance. Probably because once you start being able to do more than just play, it seems like it is something that has state and an instance seems more logical. But I could see the argument for putting that behind static methods.

I ended up going native for the project I was using this on due to unrelated memory issues with react-native so I'm no longer using it.

[timhagn]

Hi @cymen,
any plans on porting your version to RN 0.16.0?
Am not that fluent in Objective-C (and that's an exaggeration ; )
so would be pleased if you could lend a helping hand.
Have to port my app from Android to iOS soon, so
help'd be very appreciated!

Regards,
Tim.