Support URLs that have a path component.

Rather than this:

```javascript
var client = restify.createJsonClient({url: 'https://api.twitter.com'});

client.get('/1.1/statuses/user_timeline.json');
client.get('/1.1/users/show.json');
```

This Pull Requests lets you do this:

```javascript
var client = restify.createJsonClient({url: 'https://api.twitter.com/1.1/'});

client.get('/statuses/user_timeline.json');
client.get('/users/show.json');
```

It uses `path.join()` to join the two fragments so you don't need to think about slashes.

This will break existing code if people have a URL with a path set in the client.
`path` is intended for filesystem paths and will not generate a valid URL on Windows.