Add onTopHandler functionality.

[SMHFandA]

It is very usefull functionality, i think. If page compleatly scrolling up then call function. I implement this feature, but i'm not strong JavaScript programmer. Maybe you could implement this feature better. Sorry for my english.

[markgoodyear]

I like this idea. I think we can add in more callbacks, such as onBottom too. Would you be happy to add that to your PR?

Also would you be able to amend the code style so it fits in the rest of the styles (e.g. spacing around before the curly bracket, and use strict equality ===). Adding some info to the readme, and also run gulp to generate the dist files would be great too :).

[SMHFandA]

I'll done. Also i'm added demo section. See how look.