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)

So you're suggesting that I make it a `Widget` in `scene/widgets`, and make it a subclass of `Widget` instead of `Visual`?

Another benefit (it looks like) is that you get a border for free, which is nice because I wanted to add one eventually anyway.