Add basic support for data loading in yamltodb.

 * pyrseas/database.py (Database._link_refs): If present, pass data
   loading configuration to tables.link_refs.  (Database.diff_map):
   Call tables.load if dataload present in config.
 * pyrseas/dbobject/table.py (Table.load): New method to generate SQL
   to load data.  (ClassDict.link_refs): Add dataload parameter and
   process it.  (ClassDict.load): New method to iterate over tables to
   be loaded.
 * pyrseas/testutils.py (InputMapToSqlTestCase.to_sql): Add config
   param and merge it with test configuration.
 * tests/dbobject/test_static.py: New test.
Rename 'dataload' config item to 'datacopy'.
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.
Allow the repository config path to be specified in the user config file.