Lookup policies in the current namespace

Addresses #12. If the policy is not defined in `namespace`, `const_get`
will search through the inheritance change of `namespace` to find the
policy.
It seems that ``klass = namespace.const_get(klass) if klass.is_a?(String)`` will get the wrong policy if the ``object`` is namespaced which is the case for engines. Basically when ``klass`` is something like ``EngineName::UserPolicy`` and namespace ``EngineName::Admin``  ``const_get`` will not return ``EngineName::Admin::UserPolicy``.