Do not take global as default context but this for AMD distribution (should fix #323)

No idea how to test that. Would love to help. :)
I'd just leave it as

```js
var Ember = global.Ember;
if (typeof Ember === 'undefined' && typeof require !== 'undefined') {
  Ember = require('ember');
}
```

so that if there's an `Ember` global that is used.