Override Backbone.Model.clone correctly

[rjcorwin]

This has been a mistake for a long time but it seems that older versions of Backbone have been more forgiving.

An older backbone where we were lucky it worked with backbone-couchdb.js: https://github.com/jashkenas/backbone/blob/0.9.2/backbone.js#L450

A recent backbone Backbone.Model.clone where we get weird results when using backbone-couchdb.js: https://github.com/jashkenas/backbone/blob/1.2.3/backbone.js#L707

The forgiveness happens in the constructor. Even if it used to work when used incorrectly, it definitely doesn't work now and this needs to be fixed.