Include a default redis client

[nfriedly]

Description

I think we should include a default redis client, and accept a new option for the connection details that is passed directly to the redis client.

I think node-redis and ioredis are the two most popular ones. node-redis has more github stars while ioredis has more npm downloads. I'm leaning a bit towards ioredis since it has a smaller file size, although node-redis has the advantage of only accepting 0 or 1 arguments to it's constructor, whereas ioredis accepts 0-3. (We could accept an array for the 2+ arg versions.)

We should still keep the sendCommand option around for backwards compatibility and advanced usage. It should probably throw an error if both sendCommand and our new option are set at the same time.

Why

This should simplify usage.

Alternatives

Not changing anything

[gamemaker1]

I'm on the fence for this one; don't really see a need to add this option. We can fix #202 by explaining the sendCommand function better in the documentation.

However, if we do implement this, I think we should go with ioredis.