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.
Definitly :-1:  to include a PNG in the Gammapy repo.
I thought you could produce a similar plot with a script here, but if that doesn't work out nicely, then link to the IPython notebook on nbviewer with that PNG plot instead of including it here.
(and make sure you remove the commit where you added the PNG so that the repo stays small)