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.
The plot generated here doesn't look good at all (attached). Is it worth still including the code so there is an example on usage (without the actual plot it produces), and then, as you suggested, the `background_iterations.png` from here http://nbviewer.ipython.org/github/gammapy/gammapy-extra/blob/master/notebooks/plot_results.ipynb as the image as it's a nicer example?