Overlay scrollbar fix

Overlay is no longer preventing scrollbar from functioning for dialogs which height exceeds window’s height.  (https://github.com/likeastore/ngDialog/issues/94 fix)
@dzikowskw please follow code style, this lib has `tab` as indentation

Check http://editorconfig.org/ for your IDE or text editor, it's very helpful, ngDialog has its own rules which will be used automatically.