Make rocksdb_bench compile with GCC

To compile rocksdb_bench with GCC it needs libzip2.
Compile RocksDB benchmark with C++11
Use RocksDB's C++ API

As the ForstDB benchmark is C++ we can use RocksDB's C++ API directly
instead of going through the C wrapper.
Merge pull request #5 from vmx/rocksdb-cpp

Use RocksDB's C++ API