- merged sync_simplify branch

- merged sync_simplify branch
- The methodology behind "primaryjoin"/"secondaryjoin" has
been refactored.  Behavior should be slightly more
intelligent, primarily in terms of error messages which
have been pared down to be more readable.  In a slight
number of scenarios it can better resolve the correct
foreign key than before.
- moved collections unit test from relationships.py to collection.py
- PropertyLoader now has "synchronize_pairs" and "equated_pairs"
collections which allow easy access to the source/destination
parent/child relation between columns (might change names)
- factored out ClauseSynchronizer (finally)
- added many more tests for priamryjoin/secondaryjoin
error checks