Sort static data by primary key columns or default order. Fixes #80.

 Based on 27 Jan email to pyrseas-general from Kim Bisgaard,
 editorialized by me.

 * pyrseas/dbobject/table.py (Table.data_export): If table has primary
   key, create order_by list using key columns, else use defined order
   of columns.  Use new sql_copy_to to copy data out.
 * pyrseas/lib/dbconn.py (DbConnection.sql_copy_to): New method using
   copy_expert to copy to a file using SQL COPY.
 * tests/dbobject/test_static.py: Add test to verify primary key sorting.