Adds HTMLBars `loc` helper

+ ports `loc` helper to work with `htmlbars`
+ adds new tests for multiple parameters
+ makes sure to notify users that bindings are not supported yet
+ adds an `assert` function b/c `Ember.assert` doesn't support passing `function` as a second parameter. As far as production build of `Ember` goes, I think it's better to have a `function` that never gets invoked than a loop like that:

```javascript
for (var i = 0, l = params.length; i < l; i++) {
  if (params[i].isStream) {
    Ember.assert('You cannot pass bindings to `loc` helper');
    break;
  }
} 
```
idiomatically we've been checking `options.types[i] === 'id'`. I'd prefer that, though this definitely works.