WIP: mplexporter / pyplot interface

The todo list:

* [x] Translate Cyrille's code to vispy submodule + example.
* [x] Make text display.
* [x] Make `Line` based on `Visual`.
* [x] Make `Marker` a visual.
* [x] Complete support for marker properties.
* [x] Change to use `SceneCanvas`. This should facilitate:
* [x] Deal with different plotting coordinate systems.
* [x] Deal with multiple axes.
* [x] Add image support.
* [x] Complete text display options, with proper scaling and location.
* [ ] Improve image visual quality and fix view transformation.
* [ ] Complete support for line properties.
* [ ] Improve line quality (agg).
* [ ] Make it faster (it's *very* slow currently).
* [ ] Add tests.

Things that would be nice, but not 100% necessary to have things working (and probably difficult):

* [ ] Make collections work (efficiency).

Excellent -- this did indeed make it fit appropriately, at least at the initial viewbox size. MPL:

![screen shot 2014-08-10 at 5 10 26 am](https://cloud.githubusercontent.com/assets/2365790/3869270/57d24882-2087-11e4-8f01-14bb84e4117f.png)

Vispy (note the z order isn't handled, so the line isn't shown):

![screen shot 2014-08-10 at 5 11 01 am](https://cloud.githubusercontent.com/assets/2365790/3869271/6813f3c6-2087-11e4-90d0-bded97ef2194.png)

However, I think the relationship between the `Image` visual and the `SceneCanvas` is broken. If I resize the window a little bit, the `Text`, `LineVisual`, and `Marker` visuals continue to look correct, but the `Image` does not:

![screen shot 2014-08-10 at 5 11 22 am](https://cloud.githubusercontent.com/assets/2365790/3869273/74746bdc-2087-11e4-8754-671526cd616d.png)

Here we can at least see that the line is pointed in the proper direction as it appears from behind the image, so the axes are set up properly.