Allow retrieval of key by full fingerprint

  GPG 1.4.12 switches to using the full fingerprint of a key when requesting
  a refresh (commit 6fe25e5602fabe92c68e5ba30e4777221e8612df). We were only
  supporting retrieval by 32 or 64 bit key ID. Detect when we're passed a
  fingerprint and truncate it to the last 64 bits so we can look it up.
  In the future we probably want to extend to being able to do lookups by
  full fingerprint.