Code review comment for lp:~gmb/launchpad/bugzilla3.4-see-also-bug-419134

This branch fixes bug 419134 by making the BugzillaAPI ExternalBugTracker implement the ISupportsBackLinking interface.

I've added a test implementation of the Bugzilla API's Bug.update_see_also() method, which I've then used in the tests of BugzillaAPI.setLaunchpadBugId().

This branch is an rc candidate.

= Launchpad lint =

Note: I suspect that the lint notices about email.Utils are because lint runs using the system Python (2.6) rather than bin/py.

Checking for conflicts. and issues in doctests and templates.
Running jslint, xmllint, pyflakes, and pylint.
Using normal rules.

Linting changed files:
  lib/lp/bugs/doc/externalbugtracker-bugzilla-api.txt
  lib/lp/bugs/externalbugtracker/bugzilla.py
  lib/lp/bugs/tests/bugzilla-api-xmlrpc-transport.txt
  lib/lp/bugs/tests/externalbugtracker.py

== Pylint notices ==

lib/lp/bugs/externalbugtracker/bugzilla.py
    20: [F0401] Unable to import 'email.Utils' (No module named Utils)

lib/lp/bugs/tests/externalbugtracker.py
    1642: [W0108, Urlib2TransportTestHandler.default_open.<lambda>] Lambda may not be necessary
    1656: [W0108, Urlib2TransportTestHandler.default_open.<lambda>] Lambda may not be necessary