Photo downloading is more resource friendly

Fixes around SkyDrivePhoto interactions:
 - Make sure that bitmaps for downloaded thumbnails are marked as purgable
   so the system can reclaim that memory if needed
 - Change the downloading logic for photos to be done in a task to prevent
   UI thread blocking
 - Automatically scale downloaded images to the device screen dimensions
   that will prevent crashing on high-resolution images
 - Annotated the SkyDriveListAdapter.getView() to indicate that it is not
   built to be mindful of CPU and network band-width