Rubberband

This extension allows to select cells in a notebook using a rubberband style selection.
Press shift + click mouse and drag the rubberband over the desired cells:
![rubberband](https://cloud.githubusercontent.com/assets/2445216/4613466/0d5fd2f2-52d6-11e4-9dab-3d5975685b61.png)
This marks all cells touching the rubberband and adds `cell.metadata.selected=true`.

Works only for cells that are currently visible in the browser. Not sure how an UI for scrolling while dragging the rubberband  should work.