Re-export eleveldb:iterator/3

This was originally exported in
e6ae8c5abd0d41345311ae895b349a8cb31d744e,
but was accidently undone in 025a3602a55845d1f4f3940158a627772d4e9bd0.
Merge pull request #60 from basho/wrd-export-iterator-3

Re-export eleveldb:iterator/3