[riak] New YCSB binding for Riak.

The binding is based on the one created by Basho Technologies (available at https://github.com/basho-labs/YCSB/tree/master/riak).
[riak] Made compatible with the latest YCSB version.
[riak] Added strong consistency test support and made it default.
[riak] Added missing results return based on provided fields in scan/read functions.
[riak] Added TIME_OUT status return code for those transactions which take too long to complete.
[riak] Added flexible test parameter specification via customizable configuration file and command line options support.
[riak] Added test unit.
[riak] Removed unused Constants.java file.
[riak] Removed many unused function in the RiakUtils.java file.
[riak] Choosed to use executeAsync methods to resolve a bug that didn't allow the benchmark to end whenever one or more nodes of the cluster are killed.
[riak] Fixed license issues in all files.
[riak] Done some refinements to the code.