Change models and import script

Here's a quite small branch actually.

It does the following:
- add a new shape_id field which is the ID from the shapefile
- make slug unique only against its siblings
- Fix the update path bug (double save in the import script isn't nice but spent too many time try to fix it in the model code!)

Please note that when you do the schema migration, you *must* remove all the area (do a re-import) due of the new shape-id field.

We can discuss of the branch here. Next step would be to make some sanity checks maybe?