added 'page up' icon to toolbar

This is a *small* but really useful addition to the toolbar -- an icon that scrolls the window to 0,0. 

It just resets the window to 0,0 when you've scrolled all-the-way down/to the side reading a tab.

image: note the arrow on the top-right corner

![screen shot 2014-10-17 at 2 09 31 pm](https://cloud.githubusercontent.com/assets/204779/4683657/d97f97aa-5628-11e4-838a-e0a0ea67d3bb.png)