Support partial indexes. Fixes #76.

 * pyrseas/dbobject/index.py (Index.create): Process predicate if
   present.  (IndexDict.query): Retrieve indpred.
   (IndexDict._from_catalog): Split WHERE clause before USING.
   (IndexDict.from_map): Add predicate to attribute list.
 * tests/dbobject/test_index.py: Add two tests for new functionality.