Extend addons dependancies

Extend addons dependancies checklist to include:

- executable / application presence
  (example: document need `antiword' to index word document
                   bob_account_import need `pxview' to work,
                   etc...)

- required python module
  => check that a all needed python module can be loaded before trying to load the addons itself.

- manage conflict in dependancies resolutions.
  Christophe Chauvet as made a blueprint about this. Look a his blueprint more dicussion / more details.
  https://blueprints.launchpad.net/openerp/+spec/declare-module-in-conflict-with-others