New 'auto' view that updates an existing entry else creates it

This PR adds a new view named "auto" that gets the item id from the attributes (if missing a 400 error is returned), if an existing item matches it is updated with the provided values, else a new item is created. Request is submitted using POST method.

Example usage:
```
inventaire = REST(DBSession, Inventaire)

@view_config(route_name='inventaire_auto', renderer='jsonp')
def inventaire_auto(request):
    return inventaire.auto(request)
```
URLs and request body:
```
POST http://host/main/wsgi/inventaire/auto
{
    "designation": "Portable Dell",
    "equipement_id": 10002814,
    "classe": "Printer",
    "local": "PB 1234",
    "centre_cout": 49,
    "fabricant": "ARP Datacon"
}
```