[ADD] Add module project_assignment

This module allow to automatically assign user to the task when we change the stage of the issue. We can specify the partner on the stage, and overwrite the configuration in project and in the task.

This module use base_recursive_model for the configuration inheritance, currently submitted in server-tools. Check this PR : https://github.com/OCA/server-tools/pull/94
You have to indicate that the module is hosted (hopefully) on server-tools repo (and its URL).