Renamed ApcorData.from_raw_string to from_string
DownloadRequest now takes the downloader as an argument to execute
instead of in its constructor.
AsynchronousImageDownloadManager's start_downloading_workunit
method has been removed.  This functionality is now part of the model,
and the download manager just exposes a submit_request method.
Removed AsynchronousImageDownloadManager's retry_download 
method.  Just use submit_request.
Renamed AsynchronousImageDownloadManager to 
AsynchronousDownloadManager.
Extracted basic download functionality to a new Downloader class.
ImageCutoutDownloader now only handles logic related to downloading
of cutouts.
Doc cleanup.
Removed print statements.
Added "Single images" and "Image triplets" radio-button options under
the Display menu.
Created models package containing validation module.
Created exceptions module for models package.
Moved transaction acknowledging model to new transactions module.
Renamed UIModel to ValidationModel and TransAckUIModel to 
TransAckValidationModel.
Merge branch 'master' into triplet_downloads

Conflicts:
	src/ossos-pipeline/ossos/downloads/cutouts.py
Added docstrings to Downloader and exposed _fetch method as download_raw.
Removed download_object_planted method because it does the same
thing as download_raw.  Use download_raw instead.
Updated planted_mag_check.py to current DownloadRequest API.
Removed get_loaded_image_count from model (not used).
ValidationModel now has an ImageManager instead of a 
DownloadManager.  The ImageManager has a DownloadManager and the
downloaded items.  It can be expanded to manage both singlets and 
triplets.
ImageCutoutDownload.download_cutout now performs the operation
previously done by DownloadRequest.execute.