Add broadcast message system

So basically, what I did was I created a Notification with `user: nil, type: 'broadcast'` and then set the NotificationsController to return `@notifications + @broadcasts`.  The Ember HeaderController then splits the two streams and the template displays them in their respective places.  The backend determines broadcast by `user: nil` but the frontend sorts it by `type: 'broadcast'`, since it doesn't have a user field.

This isn't ideal, in fact it's Absolutely Fucking Terrible:tm:, but since broadcasts are temporary things, it won't even require a migration to improve in future.  This was just the easiest, fastest route I could think of.  The other options are a Broadcasts model or separate broadcasts.json &mdash; if either of these is preferred I can switch it with no trouble.

Anyways, in the process of implementing this I did two changes to the notification system in general:

1. The `data` hstore field is passed through with a `DS.attr()` &mdash; I hear there's some downsides to this but it didn't seem like they applied here?
2. I totally redid the HeaderController `init` function.  It was barely-modified CoffeeScript-generated shit, so I swapped it for a much simpler system.

Oh, also, previews!
![Blotter display](https://cloud.githubusercontent.com/assets/430411/3668781/8831dbca-1225-11e4-8b06-6e2c39f903e3.png)
![Admin panel](https://cloud.githubusercontent.com/assets/430411/3668783/8b3f708e-1225-11e4-82b5-a1a1ff2d6e61.png)

Basically, I want you to determine if this solution isn't too ghetto and then make sure that I didn't break the notifications sytem in the process.