Fix retweet loading for certain users

When calling:

```ruby
twitter_client.retweeted_by_user("some_screen_name", :count => 200)
```

Certain screen name values cause an empty array to be returned. Not 100% sure why, but it looks to be related to users with a lot of activity, and I'm guessing it's a bug in the twitter gem's `collect_with_count` method.

This branch gets around it by using `user_timeline` instead of `retweeted_by_user` and filtering out non-retweets afterwards.

To note: the twitter gem also implements `retweeted_by_user` with `user_timeline` underneath, but it's acting weird and doing the filtering myself is easy, so whatever.