WIP: kernel background estimation

This PR adds a first version of the iterative kernel background estimation method we implemented for [this SciNeGHE contribution](http://indico.lip.pt/indico/getFile.py/access?contribId=43&sessionId=4&resId=0&materialId=slides&confId=67).

- [ ] Remove the old version in some of the [gammapy-extra notebooks](http://nbviewer.ipython.org/github/gammapy/gammapy-extra/blob/master/notebooks/Index.ipynb#Alphabetical-list-of-notebooks) (and update to work with this new version).
- [ ] Add at least some basic tests and docs.

cc @ellisowen