Allow accessing deleted associations

Adds a `with_deleted_associations` method that returns the object in a
wrapper that checks all method calls. Those that correspond to
associations are modified such that deleted associated records are
included in the result.
Support polymorphic associations
Add documentation
Make sure *all* methods are delegated
Merge pull request #3 from zendesk/dasch/deleted-associations

Allow accessing deleted associations
Bump version & add changelog
Work with Rails 4
Add a note about compatibility