Merge pull request #194 from ChrisBeaumont/master

FIX: proper loading of subsets in GUI. Closes #189
Update travis test matrix
Astropy v0.3 compatibility. Closes #195
test against developer astropy
Merge pull request #196 from ChrisBeaumont/travis

Travis update
Enabling user-supplied matplotlib colormaps

Adding a configurable ColormapRegistry which allows the user to supply
their own matplotlib colormaps for the Image Viewer in the config.py
file. The already existing colormaps are loaded in by default.
Moving matplotlib import inside function and docstring
Simplified syntax for adding colormaps
Merge pull request #198 from jzuhone/master

Enabling user-supplied matplotlib colormaps
Travis: use conda
Merge pull request #197 from ChrisBeaumont/conda

Travis: use conda for faster installs
Initial refactor of application into frontend-agnostic base class
Use commands
Data viewers take session objects as input instead of DataCollection
Undo/redo into gui
apply_roi, add_layer now commands
PEP8
test refactor
Updated gitignore
Limit number of undos