Redis-based Persistence Doesn't Accept Database in Options

[mocheng]

* The current options include:
 *  - `port`, the Redis' port.
 *  - `host`, the Redis' host.
 *  - `password`, the Redis' password.
 *  - `redisOpts`, the options for the Redis client.
 *  - `channel`, the pub/sub channel that will be used to synchronize
 *    the various clients. Defaults to `'moscaSync'`.
 *  - `ttl`, an object containing three values:
 *    * `subscriptions`, the time (ms) after which subscriptions
 *      will expire. It defaults to 1 hour.
 *    * `packets`, the time (ms) after which packets will expire.

The Redis-based persistence has no option about "database". It would be better to have database selected by options. Then, one host & port instance can supports multiple mosca instances.

Thanks.

[mcollina]

Definitely an issue, thanks for reporting. Can you please submit a pull-request on this?

[mocheng]

Pull request #152 submitted.