Revision 3017

ordering, normalizers, booleans, documentation. details:

* changed order of table classes so that the has_a() part of a
  relationship occurs before the has_many() part.
* added WeBWorK::DBv3::NormalizerMixin, simiar to Class::Trigger, to
  manage normalizer subroutines for per-column normalization.
* overloaded normalize_column_values to all normalizers for changed
  fields.
* implemented predefined has_a_boolean() normalizer definition.
* defined boolean fields in tables using has_a_boolean().
* added/clarified docs.

still to do:

* add inflators/deflators for durations
* add triggers for setting creation dates