Support circular annotations

With es6-module-loader, it is possible to use circular imports.
Classes from circular imports are not available during the module
evaluation and thus this code fails (`Inject` is undefined):
```js
Foo.annotations = [new Inject];
```

This commit changes the annotation output to use a getter:
```js
Object.defineProperty(Foo, 'annotations', {
  get: function() {
    return [new Inject];
  }
});
```

NOTE:
If the code tries to read `.annotations` or `.parameters` property
during the module evaluation (root context), it won’t work.
@guybedford can I send you a PR for es6-module-loader to support something like this? (Please let me know if you want a different implementation/style).