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
Adds Fermi-LAT PSF data files
Adds loads script for PSF files
Adds unit tests for loading of PSF files
Adds a PSF study tutorial RST file
Adds tutorial plotting scripts for Fermi-LAT PSF study
Combined the 4 data irf files for Fermi PSF into one fits file
Updates Fermi Galactic Center data documentation for the correct IRF information