Support environments where Ember prototype extensions have been disabled

More details at: https://github.com/Addepar/ember-widgets/issues/85

Test plan: Temporarily add the following script tag to app/index.html:
```
<script type="text/javascript">
      window.ENV = {
        EXTEND_PROTOTYPES: false
      };
</script>
```
Load http://localhost:8000/gh_pages and make sure the demo page for each widget is working and does not throw any errors. 

Would love to run tests and find a way to also run them with prototype extensions disabled, but they're in a really bad state right now. Without adding tests for this I imagine we will quickly breaking compatibility again.