Adds iterative background kernel estimation code to gammapy.background
Adds unit tests for new iterative background kernel estimation code
Updates background init.py file to include background estimation code
Adds background iteratative estimation tutorial RST file
Updates tutorials index.rst file to include new background tutorial
Bug fixes in background kernel script and updates the tutorial and unit tests

update
Merge pull request #186 from ellisowen/iterative-kernel-background-estimator

Iterative kernel background estimator