Triplets and downloads

The 3-by-3 grid for the 3 image layout (#145) can now be downloaded asynchronously, building a displayable item and triggering a callback when all are finished.

The downloading API should be much more stable now, as well as simpler for general use.  For example, DownloadRequests were an artefact of queueing things up for asynchronous downloading, so now they are used solely with the ossos.downloads.async module.  

@ijiraq I have updated the example script as well as planted_mag_check.py. 

Reworked image management in model to accommodate switching between 'singlet' and 'triplet' views.  Triplet view is currently disabled because some more work needs to be done on the controller/view side.