revisions to helper:

@LaurentGoderre, please look over the changes I made. If you're okay with them feel free to merge.  Thanks for doing this!

***

**pr**

* changes error messages to mention the name of the helper
* usages `export.i18n` for compatibility with any setup

Also:

* adds tests
* updates metadata to describe Laurent as author
* updates readme/docs
I believe these need to be under the i18n variable doesn't it?

```js
var context = {
 language: 'en',
 i18n: {
   en: {key: 'value'},
   fr: {key: 'valeur'}
 }
}
```