Some Unicode tweaks for Python 2.x.

 * pyrseas/dbobject/function.py (ProcDict.diff_map): Under Python 2,
   compare stmt to both str and unicode.
 * pyrseas/lib/dbconn.py: Under Python 2, register psycopg type UNICODE.
 * pyrseas/yamlutil.py: Under Python 2, use safe_dump for yamldump.
 * tests/relation/test_relvar.py: Import unicode_literals for Python 2.