Hold shift to ignore roads

A quick solution for getting around OSM errors & bus-only roads. Just hold shift and it draws point to point.

Todo:
- [x] Leaflet captures shift when dragging a marker. Override this.
- [x] Make the notification prettier.

![screen shot 2014-06-29 at 4 38 49 pm](https://cloud.githubusercontent.com/assets/1435836/3424796/91f48ad2-ffe6-11e3-87da-692b8368dfc4.png)