move the toggle outline button

A few changes:

- give spark-buttons the ability to fire 'command' events. When their `command` attribute is set, a mouse click to cause them to fire a custom `command` event into the dom
- move the outline toggle button from hovering in the text area into the toolbar. This is in support of eventually making the outline view a separate component to the right of the editing area, rather than something that hovers over the editing area.

@ussuri 

![screen shot 2014-09-18 at 3 44 22 pm](https://cloud.githubusercontent.com/assets/1269969/4328506/5b203486-3f86-11e4-8dae-07941194e650.png)

Agree with 3, but the lines may get a bit blurry with Polymer. Let's bear with it and see where we end up.

re: 1, we should unify, and I think we should unify around commands.

Re: 2 and 4, I think those can be addressed by adding some more code and rigor around listening to commands. Right now, it's very much hard-coded into this location. I'll clean this site up a bit - in the file - with the goal of later moving some command code it into its own library.