Add prefix_id to config

It is desired that all instances of a resource model have their IDs prefixed by the same set of characters.  For example, a virtual collection model will always be prefixed by 'vc'.  Creating an instance of the virtual collection model and passing in an id='123' will produce a full id='vc123'.  This works in conjunction with base_uri to construct the full rdf_subject.

```
      class DummyResourceWithPrefix < ActiveTriples::Resource
        configure :base_uri => 'http://example.org', :prefix_id => 'b'
      end
      d = DummyResourceWithPrefix.new('1')
      d.rdf_subject.to_s
      # http://example.org/b1
```

NOTES:
* For backward compatibility, a prefix will not be added to the full generated rdf_subject, if no prefix_id is specified.
* When base_uri is not used to construct the rdf_subject, the prefix_id will not be used.