Resolving small conflict in comments
Changes to make CE tests pass.
Closing a singleton base domain now marks the variable as specified.
This seems inconsistent with a previous commit to distinguish specified
variables from variables with singleton base domains, but that commit
didn't touch Variable::handleRestrictBaseDomain, which does specify
variables whose base domain gets restricted to a singleton.  This should
be clarified and made consistent. See issue #165.

Made the beginning of the ConstraintEngine::ChangeType enumeration and
the DomainListener::ChangeType enumeration into line.  See issue #166

Added initializers to the EquivalenceClassCollection constructor, which
caused some random test failures sometimes.