[Java] Add static method RocksDB.loadLibrary()

Summary:
Add static method RocksDB.loadLibrary() which loads necessary library
files.

Test Plan:
make rocksdbjava
make jtest
make jdb_bench
java/jdb_bench.sh

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: ankgup87

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18375