Add basic support for copying tables out to files.

 * pyrseas/database.py (Database.to_map): Call tables.copydata() if
   applicable.
 * pyrseas/dbobject/table.py (ClassDict.copydata): New method to
   iterate over tables and copy their data.
 * pyrseas/lib/dbconn.py (DbConnection.copy_to): New method to execute
   COPY TO commands.
 * pyrseas/testutils.py (DatabaseToMapTestCase.to_map): Add config
   parameter, after stmts, for symmetry with
   InputMapToSqlTestCase.diff_map.
 * tests/dbobject/test_{schema, table}.py: Changes to adjust for new
   parameter to to_map, above.
 * tests/dbobject/test_static.py: New test to verify this
   functionality.