Timestamp Key

Adding a `MONGODB_TIMESTAMP_KEY` option to override the default behaviour of `MONGODB_ADD_TIMESTAMP`. 

E.g. With a setting of `MONGODB_TIMESTAMP_KEY = "created"` would add the following to item:

    {
        ...
        'created': ISODate("2013-01-10T07:43:56.797Z"),
        ...
    }

Only works if `MONGODB_ADD_TIMESTAMP` is set.