[Nitro] Join tables

[Rob Pitt]

After updating to the latest nitro dev repository I ran delete_all on a 
class that had a join table with itself. Once all records were deleted 
there were numbers left over lying around in the join table. I never 
noticed this before because I had patched the old version to add 
PostgreSQL foreign key constraints on join tables. I will put these 
constraints back into our version here which will take care of the 
problem for us and possibly look at why that happens later but I thought 
everyone else might like a heads up.