Use milestone in launchpad bug tracking system

If we define some milestone on launchpad bug tracking system, we can assign bug to spefic version.

So we can:
  - how many bugs affects which version (stable / dev / ...)
  - just change milestone, if the bug can't be fix / won't fit in the time frame for next release
    (let bug open, mean don't close as Wont Fix if this will be fixed in future release)
  - See what new feature will be implemented / which major bug will be fix in a precise version.

More info in Launchpad Help:
  - https://help.launchpad.net/Projects/SeriesMilestonesReleases#milestones

Some ideas:
  - "trunk" milestones on trunk series (for bug which we can't say when they will be fixed)
  - Keep milestone up-to-date, between projects. Some projects have milestones, some have only oudated one, some other don't have one. (openerp, openobject-addons, openobject-client, openobject-server, ...)

This will look like:

Series 4.2
   Milestone 4.2
Series 5.0
   Milestone 5.0_rc2
   Milestone 5.0_rc3
   Milestone 5.0
   Milestone 5.2

Serie trunk
   Milestone trunk