Added option to specify an existing redis client upon initialization

[david-a]

Added an option to specify an existing redis client upon initialization, added tests and updated readme and comments. Also upgraded bcrypt to avoid known vulnerabilities (won't pass npm install without this upgrade).

[matthewlilley]

@florianheinemann Would you accept a version of this? I use io redis, and sentinels in my application, which isn't compatible with this store, as there's no option for setting a custom client. There's interoperability between io redis, and the standard node-redis client, so adding this option shouldn't be a problem.