Add Redis cache implementation, prefer over memcached, tests & config sample doc

I am using redis as the session cache and also wanted to use it as the fast cache instead of memcached so here is a first version. tests and sample configuration included. If even more scalability is needed RedisArray should be implemented next.
nope, like memcached, xcache etc it is a php extension: https://github.com/nicolasff/phpredis in debian `sudo apt-get install php5-redis`