Add config option for adding :toctree: on autosummary lists.

IPython uses some lists of class attributes and methods, but we don't
generate separate doc pages for each entry, so Sphinx throws warnings
about "toctree contains reference to nonexisting document".

This adds a Sphinx option called 'numpydoc_class_members_toctree',
defaulting to True, controlling whether the autosummary lists are
generated with the :toctree: option. Setting it to False suppresses
these warnings, but prevents the lists acting as tables of contents.

Addresses issue gh-5.
Document numpydoc_class_members_toctree option
Merge pull request #6 from takluyver/class_members_toctree-option

Add config option for adding :toctree: on autosummary lists.