Drop (and reinstate) FK constraints for tables being truncated for import.

 * pyrseas/dbobject/table.py (Table.data_import): If table has a
   foreign key reference, drop the constraint before truncating and
   reinstate it afterwards.
 * tests/dbobject/test_static.py: New test to cover change.
Merge branch 'r0.7' of github.com:jmafc/Pyrseas into r0.7