Option to set custom Redis database number

Hello,

Since Redis v1.0.0, it possible to have many databases across one Redis server.

It would be great to add additional flag for writing new queue into custom database number:

gearmand -d ... --redis-server 127.0.0.1 --redis-port 6382 --redis-database 4

By default, `--redis-database` should be "0"

Official documentation: http://redis.io/commands/select
About Redis multiple databases: http://rediscookbook.org/multiple_databases.html