Merge pull request #65 from adonath/model_image_creation

Creation of model images and image measurements
Minor fixes to morphology/model.py
Add gammapy/morphology/tests/test_model.py
Added plotting.py and blue_red_colormap
Added containment radius/fraction methods.
Added gaussian_sum_moments
Added formulae to docs
Added gaussian_sum_moments function
Fixed minor bugs
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