cr housing units + evr improvements

Hi guys

Our idea is: We need to improve the shelter management (Configurable).
A shelter must be considered as a "container". Housing units must be defined within all shelters. A housing unit is part of a shelter and we can enroll evacuees in them.
The housing unit management guarantees a detailed management of shelters and evacuees.

We  don't care about the "type" of housing unit: 
If the shelter is a stadium -> housing unit is a tent
if the shelter is a hotel -> housing unit is a room etc
We care just what kind of evacuee could be accommodated.

I added improvements in evr module: added filters, added field descriptions, added report fields 

Yes, it is possible to filter the options in a drop-down field by the selection made in another field, using a jQuery script called $.filterOptionsS3. It can be applied in the controller like:

    # Filter shelter unit options by selected shelter
    options = {"trigger": "shelter_id",
                     "target": "shelter_unit_id",
                     "lookupPrefix": "cr",
                     "lookupResource": "shelter_unit",
                     "optional": True,
                      }
    s3.jquery_ready_append('''$.filterOptionsS3(%s)''' % json.dumps(options))

Note that this requires a "shelter_unit" function in controllers/cr.py that applies s3_rest_controller.

If you need to debug it, the script is defined in static/scripts/s3/S3.js