Allow other SchemaNode arguments to be passed to SQLAlchemySchemaNode
during init, allow config of Mappings produced, fix dictify issue

Read colander.Mapping configuration settings from the original mapped class, if available. Specify as __colanderalchemy__.

Allow non-sqlalchemy nodes within the SQLAlchemySchemaNode. Just ignore them within the dictify method (and log accordingly, in case unintentional).

Add tests for latest features
Merge pull request #17 from davidjb/master

Increase SQLAlchemySchemaNode configurability and minor fixes