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)

Unclear on why this is needed it looks like b93fc8f6 changed the base class from doing this to doing something a bit more complicated.