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)

The markdown one's still in the editor - nothing's changed about that. I hope to at some point move it into an actions section in the outline view.

re: 1, I can move the button to the right. It'd probably look better there.

For 3: we have tooltips. It's not really different from the situation now.

2. Agreed, however I hope to eventually refactor to where the outline view is relevant for a lot more content types. And one of the designer's mocks did show the outline button in the toolbar. I'm comfortable with it there, and - for now - only being relevant for dart content.