Iterative kernel background estimator

Made a new PR as couldn't push to the old one: #173

Still in progress. To do:
 - [x] Add unit tests
 - [x] Finish tutorial
 - [x] Include initial image and mask image additionally in the tutorial
 - [x] Clean up code formatting
 - [x] Add docstrings
(Anything else?)

Please comment if you see anything to be done or changed.
There's no clear advantage / disadvantage to put it in the Sphinx tutorials or into IPython notebooks ... you decide.
What I think is important is to have a nice example somewhere that shows that the algorithm works for Galactic Fermi data if sensible parameters are chosen.