Document the expected use case of setup_schema via SQLAlchemy event listeners
Ensure missing schema nodes map to suitable values rather than colander values
Switch default 'unknown' mapping setting to ignore (Colander default). Ensure cloning copies all kwargs too.
Ensure relationship mapped schema nodes have a name attribute
Merge pull request #19 from davidjb/master

Correct what setup_schema does, use unknown='ignore' by default, ensure 'missing' (non-required schema nodes) deserialize correctly