Add crossfade transition

- Also, now using <span> with background-image for easy background-sizing
Add basic infrastructure for keyboard commands

Also, a largely unused debug mode
Use separate queue for "new" photos

As new photos come in, add them to the new photos queue. Then, favor
items in that queue when picking things to display.
Improve debug display
Add "flash" effect when new images are shown.
Keep new photos on slightly longer

(Only applies when there's not a backlog of new photos)
Improve debug text (add position + photo count)
Merge pull request #1 from matthinz/master

Add transitions + new photo queue