Ternary

These are my minor updates to code implementing ternary projections developed by Kevin Davies in 2011 (https://github.com/kdavies4/matplotlib/tree/ternary2); my changes simply make his code work with recent versions of matplotlib.

Ternary plots are a convenient way to visualize the three-dimensional unit simplex; I'm attaching a simple example from my own research on three-player games.
![numbers_game_with_dummy](https://cloud.githubusercontent.com/assets/605452/5146006/d416256c-719f-11e4-84e9-118cbe939dd7.png)

It is still something like the idea version of vaporware, I am still working on getting everything articulated right.

This is not the right place to get into this, but the very short version is that I think all plotting calls should look something like

```python
arts = plotting_function(ax, data, style)
```

See http://matplotlib.1069221.n5.nabble.com/pyplot-OO-convergence-td44030.html for some of the discussion about this.  There is not consensus on this by any means.