Community addons

Some changes to runbot in order to better test with community addons

When testing a repo which does not contain odoo, but addons from OCA:
* Only test modules from the repo, don't test all of odoo's addons
    * opener-hr will only test hr-related addons
* Test Community addons in openerp/addons-to-test instead of openerp/addons to better separate them from official addons
* Add dependency repos field to specify additional addons to fetch
    * Some OCA repos depend on modules found in other OCA repos
    * Transform `fallback_id` of `runbot.repo` into a many2many called `dependency_ids`. Migration scripts have been added.
* Add function which will find the most likely main branch a PR comes from
    * using the git merge-base and git log commands
One more thing: could we double-check that there is actually a github token configured before we call the github API for a repo? Basically we'd skip this first heuristic in that case. This would allow us to keep supporting git repositories hosted outside github, as you were suggesting.
Thanks!