Add MongoDBFormatter

A Formatter that converts \DateTime to \MongoDate and objects/arrays recursively to sub documents.
Might be best to use `class` and to combine file/line to stay in line with what the normalizer formatter does (this also goes in the BC issue of migrating to this formatter by default).

i.e.:
```
            'class' => get_class($e),
            'message' => $e->getMessage(),
            'code' => $e->getCode(),
            'file' => $e->getFile().':'.$e->getLine(),
```

Other objects probably should use `class` as well for consistency.