Correct dropping of PRIMARY KEYs that are still being referenced.
Correct dropping of PRIMARY KEYs that are still being referenced.

 * pyrseas/dbobject/table.py (ClassDict.diff_map): Drop all FOREIGN
   KEYs separately first, then drop all other subordinate objects
   and the table itself.

perseas/Pyrseas,PJMODOS/Pyrseas,dvarrazzo/Pyrseas,reedstrm/Pyrseas