Custom type.datastoreSelect as a default prop.select

This follows https://github.com/dresende/node-orm2/pull/375

Typical example on a PostGIS table
```
orm.db.defineType('GeoJSON', {
	datastoreType: function(prop) {
		return 'geometry(Geometry, 4326)';
	},
	datastoreSelect: function(prop, helper) {
		return 'ST_AsGeoJSON(' + prop + ') AS ' + prop;
	},
	valueToProperty: function(value, prop) {
		return JSON.parse(value);
	},
	propertyToValue: function(value, prop) {
		return "ST_SetSRID(ST_GeomFromGeoJSON('" + JSON.stringify(value) + "'), 4326)";
	}
});
```