Fix Issue #24 Denormalized ForeignKey bug or incorrect declaration?

Fixes Issue #24 Denormalized ForeignKey bug or incorrect declaration?

Test added and passes in sqlite.
Reduce memory during rebuild by using iterator()

Reduce the memory used while rebuilding denorms
Merge branch 'master' of github.com:gerdemb/django-denorm into gerdemb-master
Merge branch 'gerdemb-master'