Add support for auth in redis-benchmark.

Bug fix for auth in redis-benchmark in order to close this issue: https://github.com/antirez/redis/issues/244

I am skipping steps 1 and 2 on the contributing guidelines because 1) its a bug fix, and 2) its been discussed on google groups already a few times, and also because its a fairly trivial bit of code so nothing lost if not wanted.

I was using this code to run against my servers all night for testing, and also ran with make test with no problems. If there is any other testing I can do let me know.

The downside to this change is that failure to auth will still report "ops per second" which is a bit misleading, but is no different than running against a server without this patch. Its not a regression, I just didn't know of a nice and clean way to ensure that the auth was successful before letting the benchmark continue. Let me know if you have a preferred approach so I can implemented it if needed.