Revert "Merge pull request #480 from urbanautomaton/decorator-namespace-option"

This reverts commit f93542ca7c31cfb950f64f4f19aa68d701f0c095, reversing
changes made to f4dd9e0206929a81274256daf6021b8c821f4757.
Use source.decorate when decorator is not specified

Closes #490
Merge pull request #494 from haines/sti

Support STI in decorated associations