PostGIS support

If you add a PostGIS db, you can store MapItems and return MapItems from a PostGIS database

```
// connect to PostGIS
mapstore := NewMapPLZ()
db, _ := sql.Open("postgres", "user=USER dbname=DB sslmode=SSLMODE")
mapstore.Database = NewPostGISDB(db)

// creating point on PostGIS is identical to adding it to MapPLZ
pt := mapstore.Add2(40.1, -70.2)

// updating point is identical to modifying it on MapPLZ
props := make(map[string]interface{})
props["color"] = "red"
props["mph"] = 60
pt.SetProperties(props)
```