Merge pull request #92 from cdeil/issue_92

API docs for `gauss2d` function not generated because `Gauss2D` class shadows
Merge pull request #98 from cdeil/datasets

Rename gammapy.data to gammapy.datasets
Fix import in gp-sherpa-like
Add docs/image/survey_example.py
Update image plotting example description
Add GalacticPlaneSurveyPanelPlot and fits_to_png
Install aplpy for travis-ci sphinx build
Fix minor issues with panel plotting