Support connecting to a Redis socket

[stottsan]

We wanted the option to connect to a Redis socket, in addition to the TCP port. Defaults remain the same, and passing in an array with 'host' and 'port' set still works, but now you can pass in 'socket' and connect to the socket.

Example:

$redis_server = array( 'socket' => '/tmp/redis.sock' );

Hopefully someone else finds this useful, thanks!