ENH: auto generate tags from API

[story645]

We have some rough grouping by function on the matplotlib gallery, but would be really useful if for example every example that calls Axes.pie could get tagged pie and we could basically autogenerate the initial tag cloud off of the the Axes plotting methods and probably some other key methods.

Added bonus is that we can then may populate the mini gallery off the tags or link to the tag in the API description and plot type entry for the method.