Added friendships/lookup endpoint

Suggested as per #43 (thanks @lukaswhite), implemented here. Example usage: 

```javascript
client.lookupFriendships(["twitterapi", 102718482], function(response) {
    console.log(response);
})
```

IDs, passed as integers, are treated as user_id. Usernames, passed as strings, are treated as part of screen_name. Both requests (if both apply) are then sent, and Twitter's API includes the results for both params in a single JSON block (see [my comment on #43](https://github.com/desmondmorris/node-twitter/issues/43#issuecomment-65135196) for an example).