Documentation improvements and related changes

This is primarily an update of the docs, moving to a thumbnail-based example gallery (closing #69) and making a new, simpler example for using volume-based data more prominent in the set of examples. I also tweaked a number of the existing examples, primarily to improve the choices of colors (banishing the jet colormap once and for all from the example gallery). I also unified the license throughout the package, closing #67.

The one outstanding thing I'd like to have in the docs is a page on how the hell to use PySurfer on OSX (i.e. #68). I don't feel like I have a great solution to the problem, although "use Canopy and set the ETS_TOOLKIT environment variable sounds like the best option to recommend; I did all this development on Anaconda which works, sort of, with the hack Denis shared (although I have not had to build matplotlib from source).

I ran into an issue where `io.Surface` was being used (but not imported) in the `utils` module (for the label generation function). For some reason there is an issue with doing a relative import of the `io` module. I ended up moving the Surface class to `utils`, which sort of makes more sense anyway. I expect that we can deprecate `io` in 0.5 and probably remove it altogether in `0.6`.

Since I didn't change anything that would make the docs not correspond to the 0.4 version, I'll probably push them to the main site soon, but I'm currently serving (and will continue to serve) a development version of the docs [on my Stanford page](http://stanford.edu/~mwaskom/software/pysurfer-dev/index.html)