[MERGE/RFC] Add support for Launchpad API to plugin, including 'mirror branch now' command

[Jonathan Lange]

Hello all,

Attached is a bundle that adds a command 'lp-mirror' to Bazaar, that
requests the branch be mirrored instantly.

It also adds a simple wrapper API for the Launchpad API, encapsulating
the two major operations I think we'll need:
  1. Log in to the Launchpad REST service
  2. A function, load_branch, to get a Launchpad branch object based
on a Bazaar branch

The load_branch function kind of sucks, because Launchpad doesn't
provide a way of getting a branch based on a URL. I've filed this as
bug 395076 and will start fixing that bug asap.

The bundle is missing:
  - a way to 'logout', or rather to invalidate the cached login data
  - tests

I'm also unsure how Launchpad API authentication should interact with
'lp-login', and whether the directory service should switch to use the
Launchpad API.

Your comments appreciated, particularly thoughts on UI, API design and
testing strategies.

jml