add geometry column to sites RM-1694

- Add geometry column to sites, initialized with data (lat, lon, masl)
[masl == elevation]
- All sites are set to have srid/espg=4326 (WGS-84)
- Need to switch database.yml to use postgis instead of postgresql
What is this mysterious number 4326?  Even though this is just a migration, I think this should be a named constant, both here and in the Site model, and it probably should have a comment explaining it if the name of the constant doesn't say it all.