Fix for Issue 77 / nested namespaces models fail in accessible_by

This pull request includes 

- a failing test (prior to fix) in model_adapters/active_record_adapter_spec.rb when you have to related tables that have name-spaced classes and try to determine records "accessible_by"

- a fix for the failing test in model_adapters/active_record_adapter.  The issue is that after reflection, using the class name gives a missing constant exception since the name is properly namespaced/moduled.  I changed the method used to determine the fully qualified class name in order to fix the problem.
@lowjoel You are correct, `klass` is sufficient, we are `to_s` and re-constantizing unnecessarily.

I'll update in the next release.