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`?
Is this how Poisson noise appears in optical images?

In Gamma-ray images we have an expected number of counts per pixel `n_pred` and then the noisy image is imply `np.poisson(n_pred)`, i.e. it's not added.

A formula or link to how the noise is added to the image would be good IMO (but is not essential).