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 

Consider making this a hierarchical taxonomy, see 
http://eden.sahanafoundation.org/wiki/S3/S3Hierarchy
http://eden.sahanafoundation.org/wiki/S3/S3HierarchyWidget

Example see org_facility_type in s3db/org.py (plus corresponding import in org_facility.xsl)