fix for serializer in BinData not native mongodb format

I've added a check which if the format saved is a string (yaml/json) the data won't be saved as binary
and added a native_serialize option to the backend which will allow to serialize objects in the native pymongo/mongo document format.
Didnt' add a custom serializer since there was no way to check if data is serializable without actually serializing it and pymongo don't recognize that we are sending already encoded data (data double-encoded again)

this will allow to aggregate/map-reduce results data after tasks are done!

the only parts which still needs to be tested (though I think they should work) are:
-    group save/restore
-    task childrens store/get results