Added the ability to inject redis clients into the Faye Redis engine.

This PR allows the Redis client to be injected into the Faye Redis engine, instead of having it instantiated by the engine itself (using the `host`, `port` etc options).

### Background
We're currently moving across to using Redis Sentinel. In node, we're using [Node Redis Sentinel Client](https://github.com/DocuSignDev/node-redis-sentinel-client) to coordinate our Redis connections. This library provides a client which walks like a Redis client and talks like a Redis client, but isn't a standard Redis client (as it maintains a connection to the Sentinel, listening for failover messages, etc).

### Implementation
Two extra options have been added to the Faye Redis engine: `client` and `subscriberClient`. These are intended to be used together and as an alternative to specifying the Redis connection parameters `host`, `port` etc. They should each reference a Redis client (or in our case a Redis-client-like object).