Add phenotype phase plane into flux_analysis

This pull request makes significant improvements to phenotype phase plane plots.

Using the improved solver interface, this is now much faster. Computing a 40x40 grid took under 3 seconds on my computer with only a single thread.

The plots also look much nicer. The azimuth and elevation angles  are automatically set to more sensible defaults. The default colors are also nicer, and brewer2mpl themes can be used if installed.
![phaseplane](https://f.cloud.github.com/assets/1380168/2456403/a8e951c6-af16-11e3-9103-3d2ab90d0381.png)
![phaseplanepastel](https://f.cloud.github.com/assets/1380168/2456404/a8ea17a0-af16-11e3-916f-11f8d3ea171c.png)
![phaseplanedark](https://f.cloud.github.com/assets/1380168/2456405/a8eadb7c-af16-11e3-8014-317eddf729bb.png)