Fix class attribute side effects between instances of ImportResult.

This fixes an issue reported by @adulau on Github where importing KeyA
and asking for the ``ImportResult.fingerprints`` would list KeyA's
fingerprints, and then importing KeyB and asking for the
``ImportResult.fingerprints`` would list both KeyA and KeyB's
fingerprints.

This was caused by a side effect resulting from the
``ImportResult.fingerprints`` being a class-level attribute, which gets
modified by the first call to ``gnupg.GPG.import_key()``, causing later
instances of ``ImportResult`` to retain the side effect. This commit
causes the ``ImportResult`` class to be indempotent under sequential
composition calls to ``gnupg.GPG.import_keys()``, ultimately resulting
in listing, upon each key import attempt, only the fingerprints of the
keys which were listed *that* time.

 * FIXES Issue #81
   https://github.com/isislovecruft/python-gnupg/issues/81
Merge branch 'fix/81-import-indempotence' into develop