Implement Registry.__contains__ magic method
Remove no-op DataConceptQuerySet.contribute_to_class
Implement base Validator and FieldValidator classes

The Validator class acts as a foundation for all other query processing
steps that require validation. Since everything is constructed
dynamically based on 'meta' data structures, validation is necessary
for resiliency of those structures over time.
Move comment from group to sites
Allow for string or model field for determining the simple type
Add support for Registry.__len__

Also implemented __nonzero__ so it
is not falsy.