Add riak_cs support (imported from riak_cs/bench)

[sogabe]

This patch depends on stanchion_auth in velvet. Previous pull-req was copied stanchion_auth.hrl. to the repository.

[shino]

Good eye on the typo (ibrowse_block_size in original one in cs repo) :)

[shino]

Oh, one extra comma character after ibrowse_block_size

[shino]

Sorry for late reply.

Looks perfect expect one tiny point I commented above.
I will merge if it is corrected.

Thank you very much for contribution!

[sogabe]

Thanks for your comment. I fixed an typo in examples/cs.config.sample.

[shino]

@sogabe -san:
Merged. Thanks a lot!

[slfritchie]

Mina-san, thank you for your work and review. Would it be possible to create a quick PR to remove the 'bench' subdir from the riak_cs repo? It does not make sense to have two copies of the driver.

[shino]

@slfritchie Yes. I will do it right now after one more pull request to basho_bench which add +K flag to rebar.config.

[shino]

FYI: basho/riak_cs#536