:math: directive renamed to :mathmpl:

The :math: rst role provided by matplotlib.sphinxext.mathmpl has been renamed to :mathmpl: to avoid conflicting with the :math: role that Sphinx 1.8 provides by default. (:mathmpl: uses Matplotlib to render math expressions to images embedded in html, whereas Sphinx uses MathJax.)

When using Sphinx<1.8, both names (:math: and :mathmpl:) remain available for backcompatibility.