Added callback when loading screen has finished fading out

[Meldinoor]

New to github, just trying to figure this out.

[NevilleS]

Hey, thanks for the PR!

Couple things to point out here:

• don't worry about committing changes to package.json or incrementing version numbers; we'll need to handle bumping the version and republishing to bower, npm, etc.
• since the existing onLoaded callback is called immediately after you call finish(), I think onFinished is a slightly confusing option name - more accurately, maybe onRemove or something would be more helpful to other users
• since onLoaded does fire once the "disappear" animation starts, why do you need to wait for the animation to finish? In general, I'd recommend starting your app code before the fade out completes, otherwise it's a bit irritating for users
• finally, we have pretty good spec coverage for this library, so it'd be good to write a unit test to confirm your feature works as expected and doesn't break any existing tests (I have a feeling this one does :wink: )

ps. welcome to GitHub! :smile: