Metadata source collisions

[rcarver]

The single source-specific entry in Meta does not allow for a few possibilities:

1. Same data from multiple sources (two Flickr accounts with the same photo and separate comments)
2. Same data from multiple URIs on the same source (same image uploaded to Flickr twice, with separate comments)

#16 fixes (1) but not (2). Need to try again.

These cases are pretty rare right now, but will definitely be a thing when we sync local files. Consider same image on disk multiple times, each with an xml sidecar file containing different metadata.