Support for event triggers.

 * docs/eventtrig.rst: Documentation for new feature.
 * docs/index.rst: Add new feature to index.
 * pyrseas/database.py (Database.Dict.__init__): Initialize event
   triggers dict.  (Database._link_refs): Call new ProcDict method.
   (Database._trim_objects): Add eventtrigs to list of object types.
   (Database.{from, to, diff}_map): Process event triggers.
 * pyrseas/dbobject/eventtrig.py: Feature implementation module.
 * pyrseas/dbobject/function.py (ProcDict.link_refs): New method to
   link triggers to the functions executed.
 * tests/dbobject/test_eventtrig.py: New unit tests for feature.