Provide canonical examples of mpl in web frameworks

A common use case seems to be to use mpl on the backend (via a (micro) framework) and serve the resulting {png, svg} out to the front end.  We should have in the docs examples of how to do this.

See http://stackoverflow.com/questions/28091129/what-is-the-difference-among-idioms-for-serving-a-matplotlib-image-with-flask
