``GetFingerprintsForLabelSet`` uses intersection.

This implementation needs to be cleaned up considerably, but it
should hopefully provide some breathing room.  Ultimately this will
be extracted out of the storage layer---specifically the LevelDB
hierarchy---to interface with a generic set of abstractions for a
storage system, but this will be where it remains for now.
Merge pull request #30 from matttproud/fix/get-metrics-for-labelset-semantics

``GetFingerprintsForLabelSet`` uses intersection.