Support new addons layout on github

[Georges Racinet]

The three launchpad OpenERP projects have been merged into a single github repository.
What used to be into openobject-addons and openerp-web projects is now in the addons subrepository

The workaround everybody used so far has been to respecify this separately like this :

   addons = local parts/odoo/addons

This works, but is counter-intuitive for newcomers and breaks the convention that parts/ is managed internally by buildout ; besides it's been expected from the very beginning that a single 'version' setting is enough for vanilla OpenERP or Odoo

The recipe should add that addons subdirectory automatically if found