Allow aeson >= 0.7

Related to #3, again.
> By this, user can choose what to do with big number

Not entirely: If your `AESON.Value -> Maybe BSON.Value` returns a `Nothing`, the user cannot really choose how to handle the conversion of a big number, since there is no indication where in the JSON object it is (it could be just a single number deeply nested somewhere) - only that the conversion failed.

If you want to have the user choose how to convert such cases, you'll somehow have to let them pass in the way to handle that, like the `Scientific -> BSON.Value` function I mentioned above.