Simplify API, add new RedisClient and remove Factory

[clue]

This changeset simplifies the API by adding a new RedisClient and removing the existing Factory:

// old
$factory = new Clue\React\Redis\Factory();
$redis = $factory->createLazyClient('localhost:6379');

// new
$redis = new Clue\React\Redis\RedisClient('localhost:6379');

Internally, the RedisClient works similar to what was previously called a "lazy" client, but we're getting rid of this wording altogether. This means it automatically creates the underlying connection to Redis only when it's actually needed and will close this connection after a short while when it is not in use anymore.

A follow-up PR will update the "idle" or "keepalive" time and adjust the open and close event handling as discussed in #118.

Builds on top of #128 and #87
Refs #118