add driver-mediated datasource processing; password-free connect
  * add connect/1 to erbi
  * add 3 erbi_driver callbacks for processing/validating properties and args
  * add normalize_datasource() to erbi, using new callbacks
api implementations for connect, prepare, disconnect
  * add functions to erbi and erbi_connection
  * erbi_driver gets handle_cast clause for disconnect
reasonably complete dummy driver