Support inside overflown element.

eg: new scrollReveal( {elem: document.getElementById('srcontainer')} );
Fix offsetTop
Update README.md
Update scrollReveal.js
Update scrollReveal.js
Update scrollReveal.js
Add callback option
Move callback to the options object
Add guard if no function is defined
Rename option to: complete
Merge pull request #71 from AbeEstrada/callback

Add a function to call once the animation is complete.
Merge pull request #68 from aliencode/master

Support inside overflown element