make slightly smarter about default fit parameters

I noticed that the line/column plots seemed to have weird behavior and rarely fit the center.  It looks like the problem was that it wasn't being given an initial guess.  This just tries to fix that.

Note that I couldn't really tell what to do in the moffat case, because the moffat function in `math_helpers.py` looks strange (has extra parameters and doesn't look like a typical moffat form).  So I made my best guess, but I'm not sure how well it will work.

By the way, @sosey, are you planning on eventually transitioning to `astropy.modeling` to do this?  It's there to make things like this easier...