[ADD] Add module base_recursive_model

Hello world,

Here is a module I developed for the community verticalisation (https://github.com/OCA/vertical-community) but since it's not specific to communities I think it's best to place it here.

This module manage two concept relative to recursivity in Odoo : 
-The parent_id link, which is used in many modules even in official one, but each time the concept is reimplemented. I think it's best to create an abstract class as I did here.
-Configuration inheritance, this class allow to inherit some configuration from parent object or specified object. It's kinda complicated to describe it here, I suggest you check the modules project_assignment and vote on https://github.com/OCA/vertical-community. I'll PR project_assignment today to help with the functional reviewing of this part.
Why not setting resulting dictionary ?

    config_lines = self._get_external_config(...)