Add new functions to make images with Gaussian sources

This PR adds two new functions for making images with Gaussian sources.

`make_gaussian_sources` extends the existing `make_gaussian_image` by allowing elliptical Gaussians, including optional Gaussian or Poisson noise, and allowing an optional `seed` to reproduce the random noise images.

`make_random_gaussians` generates random 2D elliptical Gaussians using `make_gaussian_image`.

Here are the docs for the new functions:
* http://www.stsci.edu/~lbradley/photutils/make_gaussians/api/photutils.datasets.make_gaussian_sources.html#photutils.datasets.make_gaussian_sources
* http://www.stsci.edu/~lbradley/photutils/make_gaussians/api/photutils.datasets.make_random_gaussians.html#photutils.datasets.make_random_gaussians


@cdeil Do you mind if I remove `make_gaussian_image` since that functionality (circular Gaussians) is covered by the `make_gaussian_sources`?
No, this isn't how Poisson noise appears in real images.  I'm not including the Poisson noise of the sources here.  It's only the Poisson noise of the background, where the expected background is `noise_lambda`.  In a real image, it would be as you say `poisson(detected counts)`, which includes both source and background.  Not sure if its really worth making that change here -- it would need to be done pixel by pixel.