Add some unit capabilities to make_* functions

This PR is an attempt to address https://github.com/astropy/photutils/issues/173.  `make_noise_image` and `make_gaussian_sources` now have a `unit` option to specify the units of the output image and `make_poisson_noise` will now return a `Quantity` array if the input image is a `Quantity` array.

I didn't modify anything to accept or generate `Quantity` parameters because unfortunately they can't be used as inputs to astropy models (that would be nice).  The workaround would be to "de-Quantify" them (i.e., `parameter.value`) and the re-Quantify the output image.  Someone else may have a better idea how to handle `Quantity` parameters, so I didn't implement it here.