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.
This line causes an error in the Sphinx build:
https://travis-ci.org/gammapy/gammapy/jobs/35494647#L1620

The problem is that you have indented the bullet list by adding a space character before the `*` characters.
If you remove those spaces the Sphinx build works.