Enhancement and bug-fix release

This is an enhancement and bug-fix release, and all users are encouraged to upgrade.

Brief summary:

• Fixed #161: Added a status attribute to the returned object from gen_key() which
  is set to 'ok' if a key was successfully created, or 'key not created' if that
  was reported by gpg, or None in any other case.

• Fixed #164: Provided the ability to add subkeys. Thanks to Daniel Kilimnik for the
  feature request and patch.

• Fixed #166: Added keygrip values to the information collected when keys are listed.
  Thanks to Daniel Kilimnik for the feature request and patch.

• Fixed #173: Added extra_args keyword argument to send_keys(), recv_keys() and search_keys() to allow
  passing options relating to key servers.

The files for this release have been signed with my code signing key:

Vinay Sajip (CODE SIGNING KEY) <vinay_sajip at yahoo.co.uk>
Fingerprint: CA74 9061 914E AC13 8E66 EADB 9147 B477 339A 9B86