expose a leveldown instance

In the same vain as #77

A levelup instance exposes a `db` property that is a leveldown
    instance. This is documented in the levelup README.

Here we expose the leveldown instance through nut and also
    work around the fact that levelup uses a DeferredLevelDown
    instance and later mutates itself to set a real leveldown instance
    to the db field.

cc @dominictarr