Add view for Friends

[julien-c]

This adds a view to display your Twitter Friends (i.e. people you're following) along with their avatars. It is slightly more complicated than AccountsList or TweetsList as more API calls are needed (first get list of user ids, then request more info about those ids).

I also added a custom UIBarButtonItem icon for this view.

I hope you'll find this contribution helpful.

Also, I'm in the process of learning both Objective-C and the iOS SDK, so please don't hesitate to comment on anything that's not done efficiently or elegantly (e.g. data structures manipulation with NSArray and NSRange, UITableView handling, image caching, memory management, etc.).