Add support for GeoPoint type

See https://github.com/strongloop/loopback-datasource-juggler/issues/191
Does it make sense to move most of this method into SqlConnector base, so that connectors provide only a function to map a single property/column?

```
PostgreSQL.prototype.mapValueToDB = function(value, type) {
  if (type === 'GeoPoint' && value) {
    return '(' + value.lat + ',' + value.lng + ')';
  } else {
    return value;
  }
}
```