Add support for login tokens

As of Bugzilla 4.4.3+ login cookies are no longer supported.

"The User.login WebService method now also returns a token argument
containing a login token which you can use in subsequent calls to
authenticate. For security reasons, this method no longer generates login
cookies. (Bug 893195)"

http://www.bugzilla.org/releases/4.4.3/release-notes.html#v44_point
http://www.bugzilla.org/docs/tip/en/html/api/Bugzilla/WebService.html#LOGGING_IN
refactored token handling

Debatable wether this is preferred. The base package now knows about the LogIn
class.