Redis optimizations

Just a few small changes:

Use hiredis (native C redis bindings) -> speeds things up
Allow you to specify a /path/to/redis.sock to use a unix domain socket instead of tcp/ip -> less overhead, faster
modified getPostsByPids which pipelines all of the hgetall requests into one multi request -> faster in redis, less tcp/ip traffic if you're using tcp/ip socket connection

- adarqui