Let me decide how to connect to redis so I can run on Heroku

AFAICT this change (or something similar) is needed in order to run ShareJS with redis on Heroku. The [redis-url](https://github.com/ddollar/redis-url) package is a little smarter about how to authenticate to redis than ShareJS' own code. 

Here is how I use it:

```javascript
var options = {
    db: {
        type: 'redis',
        client: require('redis-url').connect(process.env.REDISTOGO_URL)
    }
};
sharejs.attach(server, options);
```

For more info see https://devcenter.heroku.com/articles/nodejs#using-redis