Add a button to code examples in docs to show/hide the prompts and output.

Not sure if latest sphinx has support for this built in, but I saw it in the official Python documentation.  Quite nice.  http://hg.python.org/cpython/rev/18bbfed9aafa

    /Doc/tools/sphinxext/static/copybutton.js


Live example:

   http://docs.python.org/3.2/library/inspect.html#inspect.getcallargs