Added addGaussian, fixed addEllipseGrid and added addEllipseRandom.

addGaussian lets you add a gaussian function to image.
addEllipseGrid now handles rotations for the ellipses (although it's
not super fast).
addEllipseRandom lets you add ellipses at random angles/sizes/locations
(any combination) to the image (again, not super fast, but ok for test
images).
Added test script to run gaussian test.