Add project action item

Alexis de Lattre's MP under review at https://code.launchpad.net/~akretion-team/project-service/add-project-action-item/+merge/216224.

> This merge proposal adds the module "project_action_item".
>
>  Extract from the module description :
>
> This module adds action items on project tasks. When a user has completed the action item of a task, he can click on a button "Done with Timesheet" that starts a wizard ; this wizard will mark the action item as done and will create a timesheet line.
>
> The idea behind this module is the following : on a task, we have the timesheet lines that give you the history of the actions made for that task... but we don't have anything to store the actions that must be carried out for that action (apart from the text field "description"). So, with this module : on a task, the timesheet lines (object hr.analytic.timesheet) still store the "past" actions carried out ; the action items (new object project.action.item) in "todo" state store the future actions that must be carried out. And we have a wizard to set an action item from todo to done and generate the corresponding timesheet line.