Adding new option to TOC ( `addBottomPadding` ) which receives a jQuery selector.
When an element is clicked, TOC will check wether there is enough space on the document / window for the target to be positioned at the top of the screen. If there is not, it will add the padding it needs to do it.

Commenting test which won't work on PhantomJS
Updating logic since it was grabbing an incorrect element. Padding is now being changed when needed and calculating increments so we don't see any jumps.

jgallen23#38

Conflicts:
	test/index.html