replace _.clone with standard mixin pattern

> because of this decision, you need to be careful with object mutability. Just use ```_.clone``` to be safe.

I'm not sure this is necessary. I replaced the `_.clone` mixins with a more typical pattern using `_.extend` on the prototype.

As far as I can tell, calling `clone` doesn't actually change the behavior. Internally, `clone` is just a call to `extend`, meaning that any benefit it could provide would already be taken care of by the `extend` already being called.

The tests all pass. But maybe I'm missing something? What issue with "object mutability" did you actually run into?

*Also*

You'll see in the README that the replacement code is a bit cleaner. 

*One more thing*

There's another pattern that's cool, too.
http://ricostacruz.com/backbone-patterns/#alternative_syntax
You might like this one better than the one I ran with.