Font handling cleanup

[lukelbd]

See #94

This also re-orders the font lists in rcParams['sans-serif'], etc. and makes TeX Gyre Heros the preferred open-source font rather than DejaVu Sans when proprietary fonts like Helvetica, Arial, etc. are not available.

[lukelbd]

This will also add the fallback_to_cm Figure kwarg. This is used to "toggle" the rcParams['mathtext.fallback_to_cm'] option figure-wide, and required because this option is only read at drawtime (it is read when the BackendBase is instantiated). A plot.rc.context loop would fail because drawing is only triggered by the ipython post-execute hook.

We use this to disable "fallback" for the show_fonts display so it is clear which fonts are missing characters... since most users most of the time would prefer graceful fallback, I wanted to make this figure-specific.

This option won't take effect until I (shortly) merge another branch that implements backend/preprocessor improvements. I could not separate the hunks.