add a 'quicklook' tool to the projection class using aplpy
FITSFigure.show...
only make FITSFigure once
add changelog entry
Merge pull request #131 from keflavich/quicklook

Add a 'quicklook' viewer for slices
add a note about broadcasting masks
update changelog and add some more examples to the docs
update the yt docs to use the new API (with warning about possible
interpration amibiguity in the pixel coordinates)