Merge pull request #17 from davidjb/master

Increase SQLAlchemySchemaNode configurability and minor fixes
Merge pull request #18 from davidjb/master

Document (almost) everything + change setup_schema function
Document the expected use case of setup_schema via SQLAlchemy event listeners
Mapped schema should use unknown='ignore', like default Colander nodes. Ensure cloning copies all settings
Ensure missing schema nodes map to suitable values rather than colander values