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)

Actually this can't really be set to use point sizes. The scale factor must be known in order to know how many rows and columns should be used in the console, but that cannot be known until the first `draw` call. And if multiple canvases with different DPIs used the same `console`, you'd be in big trouble. I'll change it to `font_scale` though, to closely mimic but still be different from `font_size` in the `Text` visual.