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).

@lcampagn it doesn't seem to work right. This is what I get:
![screen shot 2014-08-09 at 8 05 29 pm](https://cloud.githubusercontent.com/assets/2365790/3868364/460aa4ec-203b-11e4-9a19-9df721b33a71.png)

And when I resize the window, the visual doesn't stay that size. When I shrink the window size, the graphic seems to shrink at about 2x the rate it should. When I increase the window size, it grows at about 2x the rate. Any ideas? It seems like the `Image` visual might be broken...