Merge pull request #54 from ellisowen/cube_to_image_correction

Swaps round the if statements in cube_to_image to fix bug
Add tev_spectrum to data
Fix test_sample_sphere
Add sky_to_sky to celestial.py
Use astropy.table.Table instead of atpy.Table
Working on HEALPIX reprojection.
Add bin_events_in_cube utility function
Add gp-cube-bin command line utility
Import pytest from astropy
Add examples/estimate_significance_image.py
Update background/maps.py
Update background ring and image utils
Implementing the fit statistics module ...
Move examples/sherpa to dev/sherpa
Add README to dev folder
Add xspec_stats.py (work in progress, untested)
move sherpa stats tests to sub-folder
Add sherpa model_component_check.py
Add sherpa disk and shell morphology model implementation and tests.
Finish Sherpa morphology model implementation and unit tests.