Use _reference instead of reference for DS.Model

Models using `reference: DS.attr('string')`, and the error is kind of hard to debug (happens later, when using the model instance).

Since `_reference` is internal it makes sense to prepend it with an underscore. Hopefully I've made the correct changes.

Action plan:
1. If this sounds good, squash the two commits (or ask me to do it) and merge.
2. If you want to keep the current attribute name, keep the first commit (which adds a warning), throw away the second and merge. 

*I think option 2 is less desirable, but better than the current situation where there is no warning when using `reference` in your models.*