Refactored aspect fill pic of day thumb retrieval code

Renamed PictureOfDay object to AspectFillThumbFetcher. It now uses promises and has only fetchThumbnail and fetchPictureOfDay public methods.
Updated bundled cache files so they work w/changes from last commit

Also added ".dict" extension to cache files as a reminder that they are simply serialized dictionaries.
Shows pic of day in reverse chron order. Other transition fixes.

Ensured default bundled image shows only initially and then not again until other images have shown.
Shortened display interval for intially shown image (because it didn't transition from a previous image it appeared to be onscreen longer than later images.)
Refactored pic of day cycling timer logic into own object
New day's image downloaded if midnight happens while on login page

Previously you had to leave the login page and come back to it or restart the app before it would download the new day's image.
Made full screen view download appropriately sized image

The image will also pinch zoom correctly whether its the full-res yet to be uploaded image, a downloaded scaled down image, or even an image which is smaller than the device dimensions. All cases do a perfect aspect fit and will return to intial fit upon double-tap.
Merge pull request #88 from montehurd/picOfDayRefactor

Pic of day refactor and bug fixes