mv-iterator-prev branch

This branch is to address issue:  https://github.com/basho/eleveldb/issues/52
 
The async NIF release knowing disable the iterator "prev" support.  Implicitly it also disabled all "next" / "prev" combinations.  The assumption then was the Riak only used "fold" ... so don't sweat the details.  However, other folks use eleveldb outside of Riak and need the support ... this restores the feature.