Get cython functions looking nice in docs

This combines @larrybradley's branch https://github.com/larrybradley/photutils/tree/test-cython-apidocs with documentation tweaks to get Cython functions documented correctly (following astropy/astropy-helpers#43).

Note that the addition of ``autodoc_docstring_signature = True`` to conf.py doesn't seem to be necessary... In fact, it seems to even work if I do ``autodoc_docstring_signature = False``! But it seems safer and more explicit to do this just in case it's sphinx version dependent or something.