[BUGFIX beta] Do not presume extend is Ember's extend

The container cannot presume that extend is Ember's own extend function. Prototype and mootools extend native types (like functions used for templates) with the extend property.

@stefanpenner checking `__ember_meta__` is definitely not ideal, but if we are to rely upon detecting Ember-extendable objects I don't see a better way. An alternative would be to make extendability configurable, but, ugh.

Fixes #4554