Consistently handle values that are nil and NSNull

@jnjosh Please review.

TWTValidator, TWTKeyValueCodingValidator, TWTCollectionValidator, and TWTKeyedCollectionValidator no longer validate nil or NSNull. TWTBlockValidator, TWTCompoundValidator, and TWTKeyValuePairValidator rely on blocks or other validators to perform their validation, and thus have no change in behavior.
Should this be keyed as ValueValidator still?