Use GnuPG status-fd codes to determine signature validity.



 * ADDS two compiled regexes, `launcher.gnupg_goodsig_pattern` and
   `launcher.gnupg_validsig_pattern`.
 * CHANGE the signature validity check in `Launcher.verify()` to be more
   robust and accurate.
 * ADD check that the signature was created by the key with fingerprint
   which we expected it to be from.
 * FIXES micahflee#147

NOTE: This patch depends upon some of the changes in my previous patch
      for issue #137 (pull request #148) in commit
      b337b3d.