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
adds extra numpydoc_shinx_ext variable to args namespace
changes test_with_numpydoc to use imp.load_source

Should be robuster than previous solution
uses numpydoc_sphinx_ext now
adds numpydoc_sphinx_ext to default arguments
changes execution of system_test script back to regular call
fixes pep8 issues
Merge pull request #27 from cpaulik/add_numpydoc

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