Object resource

This pushes AT to 0.3.0, in the process, it completes the removal of ObjectResource and makes it possible to pass an existing AT::Resource class to an RdfDatastream.

Removing the singleton_class.properties stuff was key to getting running on AT 0.3.x.

There is a text reverted to pending for discussion. AT's behavior has changed slightly with respect to how it handles property names conflicting with existing methods on Resource.  This makes it impossible to override `type` in the relevant test.  AT should handle this better (see: https://github.com/no-reply/ActiveTriples/issues/44), but we should also discuss to what degree this is a backwards compatibility issue.
@no-reply raise an error if you're using a property called `type` with dc:type?