Restangular storage improvement

- **Adding a `getFirst` method to `restangularStorage`**: It allows to get back the first n result of a query only (instead of the complete list). For instance, if you want to get back only the last message.
- **Adding a `copy` method to `restangularStorage`**
Est-ce que ton `result` est toujours un `Array` ? Genre restangular, ne renverra jamais null or smtg?