Made fitsviewer into a package.
Split the main fitsviewer module into several smaller modules.
Renamed baseviewer to singletviewer.
Extracted base matplotlib viewer class.  The original viewer is now called
the SingletViewer.  Skeleton of a TripletViewer added.  Renamed 
view_image to display so that it is more generic and can apply to displaying
triplets of images.
Created DisplayableImageSinglet class which knows how to render itself.
Reworking fitsviewer.  Created DisplayableImageSinglet class which knows
how to render itself, has an interaction context, etc.  The main image 
viewer class now just manages these objects instead of doing the rendering
itself.  This change also gives each image its own figure, meaning they
can be cached.
Cleanup in fitsviewer package.
Model creates displayable items when images are loaded.  These 
displayable items are now used as the input to the image viewer.
DisplayableImageSinglet no longer creates figures/axes until rendered.
Moved fitsviewer to be a top-level ossos package.
Added utility function to fitsviewer package for display an astropy 
HDUList with matplotlib.
Merge pull request #172 from drusk/displayable_items

Closes #141.