Minor docstring fixes
Adds main catalog to image scripts to gammapy.image
Adds unit tests for newly added catalog to image scripts
Adds tutorial and supporting example scripts for new catalog to image methods
Updates tutorials index.rst file for newly added catalog to image tutorial
Makes small fix to units in flux column in populaltion.simulate to enable sphinx to build docs
Updates gammapy image __init__.py file for new image to catalog additions
Merge pull request #160 from ellisowen/image_from_catalog

Image from catalog
Fixes circular import bug with spectrum.flux_point
Implements SED from GammaSpectralCube code
Adds unit tests for SED from GammaSpectralCube code in gammapy.spectrum.sed
updates __init__py
Adds rectangle mask method to gammapy.image.utils and unit test