feat(service): add possibility to translate a set of translation ids

This enriches both `$translate()` and `$translate.instant()` with the
possibility to call them with an array rather than a string. This makes
the API more comfortable in some situations.

```javascript
// for example...
$translate(['FOO', 'BAR']).then(function(translations){
  console.log(translations.FOO);
  console.log(translations.BAR);
});
// ... and
console.log($translate.instant(['FOO', 'BAR']).FOO);
console.log($translate.instant(['FOO', 'BAR']).BAR);
```

Resolves #444