Support LEAKPROOF qualifier for functions. Fixes #57.

 * pyrseas/dbobject/function.py (Function.create): Add LEAKPROOF to
   CREATE statement.  (Function.diff_map): Generate ALTER FUNCTION
   statements for leakproof changes.  (ProcDict.query): Retrieve
   proleakproof.  (ProcDict._from_catalog): Use QUERY_PRE92 for
   version prior to 9.2.
 * tests/dbobject/test_function.py: New tests to verify LEAKPROOF.