Added new binding for Riak, based on the one created by Basho Technologies (available at https://github.com/basho-labs/YCSB/tree/master/riak).

Change made to the original version:

• Made compatible with the latest YCSB version.
• Removed unused Constants.java file.
• Added strong consistency test support.
• Done some refinements to the code.
• Removed many unused function in the RiakUtils.java file.
• 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.
• Added TIME_OUT status return code.

To do:

• Add different LOG errors (i.e. insufficient_vnodes_available, ring_not_ready, etc.) - for references, check page http://docs.basho.com/riak/latest/ops/running/recovery/errors/#Errors-and-Messages
• ...