ENH: Add console visual

Ready for review/merge.

Closes #418.

Example output:

![screenshot from 2014-09-02 10 27 45](https://cloud.githubusercontent.com/assets/2365790/4122587/1b455534-32c6-11e4-9821-e0ca0e949bd6.png)

I think waiting until `draw` to determine the scale factor is the correct behavior, because the visual is required to use the `event` argument to determine its DPI.