Add a bottom-buttons container to the main toolbar

This adds a second `.buttons` div to the `#main-toolbar` div that additionally has a `.bottom-buttons` class which positions it at the bottom, hovering the same distance from the bottom edge of the window that the main buttons container hovers from the top edge. The default icon position looks like this: 

![bot](http://f.cl.ly/items/1q1G1S1I440t0w3u2j12/Screen%20Shot%202013-08-23%20at%2010.48.01%20AM.png "bot")

CC @peterflynn @larz0 @njx