mongodb native serialization and group results meta collection

a little more flexible approach on storing data effectively on mongodb allowing to map-reduce/aggregate later:
-   added a groupmeta_collection option to save group results in a different collection than results
-   added a 'bson' fake kombu encoder to allow pymongo to serialize natively data in mongodb
-   modified the way groups are saved/rebuilt to avoid serializing the whole object but just the id and recreating the AsyncResult object(s) when restoring the group.