added URL support to createClient

Hi,

I thought it would be useful if we could specify redis server with URL string, like below:

```javascript
// specify remote redis service.
require('redis').createClient('redis://user:password@redis-service.com:6379/');

// or we can also specify localhost.
require('redis').createClient('//localhost:6379');
```

This overloaded interface would be useful especially for node apps hosted on heroku, on which redis add-ons is usually provided via ENV variable.

```javascript
require('redis').createClient(process.env.REDISTOGO_URL);
```