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`?
Actually thinking about it some more, I'd prefer to have a separate function `add_noise_to_image` ... have one function do one thing (which is sampling Gaussian sources onto an image in this case).

It will add another line for the user, but I think that's OK (lines have to be broken anyways if there are too many options for a single function).

`scikit-image has [skimage.util.random_noise](http://scikit-image.org/docs/dev/api/skimage.util.html#random-noise) ... I haven't looked at what they do.

But it's just a suggestion ... from your comments above I gather you prefer having fewer functions that have more parameters.