Pin paste div to bottom of screen to avoid scrollbar

In some cases, adding an absolute div to the bottom of the document
enlarges the document by 1px (thats's the height of the div) causing
a scrollbar to appear where none is necessary. By pinning the div to
the bottom, that 1px is inside the document.
Merge pull request #123 from oerpub/avoid-scrollbar-pasteplugin

Avoid extra scrollbar in github-book.