Support INSTEAD OF triggers on views.  Fixes #50.

 * pyrseas/dbobject/table.py (View.to_map): Map triggers if present.
   (ClassDict.from_map): Map triggers on views.
 * pyrseas/dbobject/trigger.py (TriggerDict._from_catalog): Handle
   INSTEAD OF in trigger definitions.
 * tests/dbobject/test_trigger.py: Two new tests.