Crash in base_external_referentials's "Add all fields": missing sequence

On an empty openerp 6.1 DB with base_external_referentials and project freshly installed, I tried to perform what I think is the generic mapping addition process, that is:

- Create a record in External Referentials
- Create dummy version/type/category
- In Objects Mappings, click Create.
- Select the "Project" model
- Click on Add all object fields

At this point, I would get an integrity error talking about a missing sequence field. By adding a sequence field in external_mapping.add_all_fields(), I fixed the problem. I'll push a branch shortly.