Add support for models that are contained by modules

This is a quick patch to allow eav_hashes to handle models that are contained within modules (e.g. Customer::Address). This is intended to support namespacing, not STI, though maybe it's a start toward STI support. I can't say for sure that I didn't break anything, since there are no tests (and unfortunately I didn't have time to write very many new tests), but it's been working well for me with both top-level and namespaced models.