Added in a couple lines that allow for h1 and h2 to be undone when a selection is an h1 or h2 and the button is clicked, similar to the functionality of the bold, italics and underline buttons
Update jquery.notebook.js

Edited double quotations to be single quotations to keep with the style.
Merge pull request #40 from TrevorHinesley/feature/h1_h2_undo_on_button_click

H1 and H2 can now be undone on button click. Fixes #31