Make uniqueness checks work with mongoid-paranoia
Also make for_slugs queries unscoped
Revert "Also make for_slugs queries unscoped"

This reverts commit 183deb8a215401a85e2afa3f8a0102de8a0633de.
Merge pull request #120 from loopj/mongoid-paranoia-uniqueness

Make uniqueness lookup "unscoped"