Support for Redis authentication

I added new feature for supporting authentication for Redis, because I am running it on Heroku and using RedisToGo as I Redis provider. 

I don't have any real experience with Lua so it might not be the most beautiful implementation. ;) 

btw. There is one quirk when running it on Heroku and using Redis host name not ip address. 
I had to define resolver in nginx config ( http://wiki.nginx.org/HttpCoreModule#resolver ). 

And thanks for great module!