Added Controlbar to UI

Added the control bar described in this [document](https://docs.google.com/document/d/1meXJaS1OdkrvC4-veCE4w5o9O9pP0tHO-7PsIzHTCt0/edit?usp=sharing), and needed for resolving [issue 500](https://github.com/hypothesis/h/issues/500) (since Always-on-highlights needs a menu space from which to be toggled).

The interface looks like this when you hover over the speech bubble:
![controlbar](https://f.cloud.github.com/assets/521978/839254/50c01100-f33d-11e2-9557-48b04f243c33.jpg)