Score photos by pair-wise comparisons

This should be supported:
1. Pick a group of photos, by tags or whatever.
2. Present a full screen UI with:
  * A top list of thumbnails of photos along one screen edge.
  * Two photos to compare, and a "don't know" button.
3. When the user has made enough comparisons (there should be a countdown), there should be some way of tagging the best photos from the top list as "favorite".

Reference: http://www.pickthehottie.com

Comparisons should be randomized. Candidates should be picked from all photos except for the most compared ones. Each photo must get at least 5 comparisons. Photos get 100% for winning, 0% for losing, and a weak 50% for N/A.