Add a create room button to the UI

The button will be visible on the lobby, and also when in a room (as you
might still want to create a new room while in another room)

In the lobby
![image](https://cloud.githubusercontent.com/assets/1035315/3634290/f389efa0-0f20-11e4-8ae5-86f7ec52ccd7.png)

In a room
![image](https://cloud.githubusercontent.com/assets/1035315/3634292/018511ca-0f21-11e4-81ce-a6948a37251b.png)

Creating a new room
![image](https://cloud.githubusercontent.com/assets/1035315/3634389/e2bb3ba8-0f29-11e4-9f00-c88fdd4fc03f.png)

Without entering a room name
![image](https://cloud.githubusercontent.com/assets/1035315/3634390/fcefc44e-0f29-11e4-8d2b-87b108bfb3ec.png)

Entering a room name
![image](https://cloud.githubusercontent.com/assets/1035315/3634395/24200d76-0f2a-11e4-934e-b19c8050e48e.png)

![image](https://cloud.githubusercontent.com/assets/1035315/3634397/2abbb662-0f2a-11e4-86b3-08cef041f22b.png)

Trying to create a room that already exists
![image](https://cloud.githubusercontent.com/assets/1035315/3634398/3fc6bc64-0f2a-11e4-80af-b326aed7f88a.png)

Fix #1154.
Maybe bikeshedding, but the first one looks better to me - it keeps it in the object initializer syntax and doesn't make assumptions on the other bits of the viewmodel.