Update bugzilla hook to use login tokens for versions 4.4.3 and above.

The service hook for Bugzilla no longer works as of Bugzilla 4.4.3.

The [release notes](http://www.bugzilla.org/releases/4.4.3/release-notes.html) state that the `User.login` method no longer generates login cookies. These cookies were what allowed method calls like `Bug.add_comment` to work without passing anything extra for authentication. The notes also mention that a login token is now returned and that this is what should be passed to subsequent calls.
(See also: [Bugzilla API Documentation](http://www.bugzilla.org/docs/4.4/en/html/api/Bugzilla/WebService.html#LOGGING_IN)).

This PR stores the token from `User.login` and wraps `xmlrpc_client.call` into another method that adds this token to the argument hash if it was actually received. This should continue to work with prior versions of Bugzilla.