Support for CLUSTER, yamltodb side. Fixes #58.

 * pyrseas/dbobject/constraint.py (Constraint.add): Add CLUSTER if
   applicable.  ({PrimaryKey, UniqueConstraint}.diff_map): Add CLUSTER
   or ALTER TABLE SET WITHOUT CLUSTER as needed.
   (ConstraintDict.from_map): Process cluster attribute.
 * pyrseas/dbobject/index.py (Index.create): Add CLUSTER if
   applicable.  (Index.diff_map): Add CLUSTER or ALTER TABLE SET
   WITHOUT CLUSTER as needed.  (IndexDict.from_map): Process cluster
   attribute.
 * tests/dbobject/test_{constraint, index}.py: Several new tests for
   clustering and unclustering.