tweet_with_user for search and timeline

Couple of comments:

- As you've noted for some of the calls, such as Search#favorites, the fixtures reflect the default API calls in which an omitted `:trim_user` option is equivalent to `:trim_user => false`. For these particular calls, I've set the :trim_user option to only appear if it has been explicitly set by the user. I had thought about just changing the specs, but you had indicated some interest in moving to VCR, which was the next feature branch I was going to tackle, and so I figured I'd update all the specs at that point.
- the `-U` option isn't on all calls that return Tweets, particularly in the `Stream` class. As you said in another thread, you weren't sure :show_user was a good idea as the data is pretty much unseeable in all usecases except when the user is piping to a text file. I think that's a good judgment call. I wonder if the path forward, then, is to create a new type of shorthand call in which the Tweet is displayed with some brief user information. 

So instead of this, sans user info:

       @johndoe
       Pushing the button to launch the site. http://t.co/qLoEn5jG

With the -U option (and without `--long`):

      @johndoe (1,242 followers, 7 years old)
      Pushing the button to launch the site. http://t.co/qLoEn5jG


I still think user data provides some interesting context to each Tweet, though I realize if you are watching a stream, you're just casually consuming it and not really caring about the identity of the Tweeter. I'm thinking of people who want to actively filter noise...for example, showing only (or highlighting) Tweets that *don't* look like they come from a spambot (the user would have their own heuristics, presumably). I realize that any *real* sustained filtering would require just writing a straight up program, though, and most users of the command-line may not require that much granularity.