add func_deriv to custom_1d_model

The documentation for `custom_model_1d()` says that it accepts `func` and `func_deriv` as parameters (see: http://docs.astropy.org/en/latest/api/astropy.modeling.functional_models.custom_model_1d.html), but the code in master does not reflect this. Here's a quick fix to allow both. I can add a test but I wanted to raise this issue ASAP.