[Java] Add iterator JNI bindings

Summary:
1. Add Java bindings for iterators
2. Unit-tests for iterators

make rocksdbjava
make sample

Reviewers: haobo, yhchiang, sdong, dhruba, rsumbaly, zzbennett, swapnilghike

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18117