evaluates RDBC URL to selects a Driver. no instance. class methods only.
registerDriver(driver) register a Driver. RDBC user should not call it. Driver#initialize calls.
deregisterDriver(driver) deregister a Driver. RDBC user should not call it.
getConnection(url, prop) getDriver and thedriver.createConnection.
getDriver(url) find a driver with url. returns nil if not found.
getDrivers returns an array of all registered drivers.