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)

Added `command` to the attributes.

I'd like to explore what a more command based arch. would look like. This is a very simple start in that direction. It might look something like this:

- one or more key bindings are bound to `'toggle-command'`
- a spark button is bound to `'toggle-command'`
- a command handler is registered for `'toggle-command'`
- when any of the triggers happen (a key or button press), the command handler is looked up by name and invoked
- the user would have a mechanism to invoke a command directly by name (say, a command bar)
- the user would also be able to re-define what keybindings invoke what commands, and what commands actually perform