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 think it would be clearer to add an additional variable to the namespace like `numpydoc_sphinx_ext` instead of overwriting `args.numpydoc`. It can be confusing when the purpose and even the datatype of a variable is changing during runtime.