Image width and height calculations were backwards (numpy arrays shape
returns num_rows, num_columns).
Added DisplayableImageTriplet class which is able to render a 3-by-3 
grid of HDULists.
DisplayableItems now have a width and height property.
Moved cutouts module into the downloads package and merged it with 
the downloaders module.
Moved get_ccd_num, get_extension and is_inverted methods from 
DownloadableItem to SourceReading.
Renamed DownloadableItem to DownloadRequest.
Added downloader as parameter to DownloadRequest.
DownloadRequest now has an execute method and is responsible for
 putting together a DownloadedFitsImage.  The downloader's API is now
more general, just requiring a SourceReading and a focal point
Split downloads module into requests and data modules.
Moved ImageReading class to ossos.download.data module.
Removed 'in_memory' parameter for DownloadedFitsImage.  It is now 
always stored in memory only, until it is needed on-disk for something 
like daophot.
ImageCutoutDownloader now directly returns an astropy HDUList when
downloading a fits file.  When downloading apcor data, an ApcorData
object is returned directly.
Merged/replaced DownloadedFitsImage class with ImageReading.
Download finished callback now just passes the downloaded object as
the parameter.
Inlined methods.
Renamed ImageReading to SourceSnapshot.
Removed model's get_current_image (use get_current_snapshot instead).
Added some TODO notes.
Renamed ossos.download package to ossos.downloads.
needs_apcor, focal_point and callback parameters of DownloadRequest
are now optional (and re-ordered).  If focal_point is not specified the 
source point in the provided reading is used.