adds command line switch --with-numpydoc as suggested in issue #26

To add the numpydoc module the placeholder ${numpydoc} is added to
the extensions variable in sphinx_conf.template.
It is set to an empty string by default and to
", 'numpydoc'" if --with-numpydoc is specified.

- added tests for cases with and without numpydoc
- added sphinx as dependency for test runs
- updated documentation
I wonder if this manipulation of the `sys.path` and importing is really safe. Does it also work under MacOS X and Windows? Could you use `imp.load_source` instead as done in `info.project`? As think this is safer and free of side-effects.