Add lookupUsers

Hi!

I've added the `lookupUsers` that takes an array of Ids as inputs, and returns the user profiles.
This would solve:  https://github.com/desmondmorris/node-twitter/issues/24

It can be tested with:

```
twit.get('/friends/ids.json', function(r) {
  twit.lookupUsers(r.ids.slice(1,10), function(users) {
    console.log(users)
  });
});
```