- column_property(), composite_property(), and relation() now

- column_property(), composite_property(), and relation() now
accept a single or list of AttributeExtensions using the
"extension" keyword argument.
- Added a Validator AttributeExtension, as well as a
@validates decorator which is used in a similar fashion
as @reconstructor, and marks a method as validating
one or more mapped attributes.
- removed validate_attributes example, the new methodology replaces it