Increase contrast and switch default font for tutorials

This PR increases the contrast in the default matplotlibrc used in tutorials, and switches back to a more modern-looking font (resolves #66). Of course such things are subjective, but I feel strongly that the current style emphasizes aesthetics (nice, soft-looking) over legibility, particularly for anyone with less-than-perfect vision.

Old style
![old_plot](https://cloud.githubusercontent.com/assets/3639698/4937545/52416858-65c4-11e4-9bae-5ea2a8a6057b.png)
New style
![new_plot](https://cloud.githubusercontent.com/assets/3639698/4937546/53c2be84-65c4-11e4-9ceb-f7296618bcd0.png)