Iterative kernel background estimator

Made a new PR as couldn't push to the old one: #173

Still in progress. To do:
 - [ ] Add unit tests
 - [ ] Finish tutorial
 - [ ] Include initial image and mask image additionally in the tutorial
 - [ ] Clean up code formatting
 - [ ] Add docstrings
(Anything else?)

Please comment if you see anything to be done or changed.
You need to move the `scipy` import into the function where it's used, because `scipy` is an optional dependency of Gammapy.

The reason this didn't fail on travis-ci is because you still have to add this line to `gammapy/background/__init__.py`:
```
from .background import *
```