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.
As for making es6-module-loader a Traceur dependency. I don't think so - you don't need es6-module-loader when using "commonjs", "amd" or "inline".

I'd like to make this a bit more easier to understand though, creating a separate issue - see #1408.