Allow for setting defaults on the prototype of objects extending

`Backbone.Form` wants defaults such as template to either be defined on the constructor or as an option at initialization. It completely ignores them when defined on the prototype.

As the Backbone convention is to define things like `template` on the prototype, many people try and fail.. I've seen a couple issues like #226 as a result of this. This PR attempts to alleviate some of that pain by allowing defaults to be set on the prototype.

Thoughts?