add etcd

[tjungblu]

Hey there,

Thanks for porting this tool to Go! I've been spending today to wire up etcd to further benchmark it, let me know what you think.

Cheers, Thomas

[dbsid]

@tjungblu Could you please add the info of supporting etcd in the readme file https://github.com/pingcap/go-ycsb/blob/master/README.md

[tjungblu]

just did @dbsid, anything missing there from your side?

[dbsid]

lgtm