Handle KEYREVOKED status message during signature verification.

If we get a VALIDSIG, but the signing key has since been revoked, we
(mostly) ignore the KEYREVOKED status message (except to append "key
revoked" to the `Verify.status` string).

 * ADD ability to handle KEYREVOKED status to `gnupg._parsers.Verify`.

 * FIXES Issue #66
   https://github.com/isislovecruft/python-gnupg/issues/66

 * THANKS TO Tom Galloway for finding this bug.
   https://github.com/tomgalloway

 * CLOSES PR#38
   https://github.com/isislovecruft/python-gnupg/pull/38
Merge branch 'fix/66-keyrevoked' into develop