Change path to MathJax CDN

See http://www.mathjax.org/changes-to-the-mathjax-cdn/ for details
ENH : if explicitly passed labels draw them by default

If the user passes in labels as a kwarg, default to drawing them
(but don't if `with_labels` is explicitly False).
Merge pull request #1256 from tacaswell/with_labels_fix

ENH : if explicitly passed labels draw them by default