OnLoad callback

[tulsileathers]

as discussed in #40

[prabhasp]

Please merge :)
I've started using react-sound recently, and onLoad is a very useful feature to have. In my case, I sometimes re-render a Sound object referring to a file that was already loaded by sound-manager2 before, and onLoading doesn't even get called on those re-renders, whereas onload does, as I see in the console.log.

[tulsileathers]

@leoasis

[leoasis]

Sorry for the delay, been quite busy. Thanks for the contribution!