Revision 1646

Basic loading of sessions containing postgis connections:

* Thuban/Model/load.py (LoadError): Add doc-string
(LoadCancelled): New exception class to indicate a cancelled load
(SessionLoader.__init__): Add the db_connection_callback parameter
which will be used by the loader to get updated parameters and a
password for a database connection
(SessionLoader.__init__): Add the new XML elements to the
dispatchers dictionary
(SessionLoader.check_attrs): Two new conversions, ascii to convert
to a byte-string object and idref as a generic id reference
(SessionLoader.start_dbconnection)
(SessionLoader.start_dbshapesource): New. Handlers for the new XML
elements
(load_session): Add the db_connection_callback to pass through the
SessionLoader

* test/test_load.py (TestPostGISLayer, TestPostGISLayerPassword):
New classes to test loading of sessions with postgis database
connections.