Hotel challenge adapter pattern

@jsteiner, @gabebw
Why does a method called `exist` print to the screen? How about calling this method `information` instead, and outputting in `hotel_database` instead of in `NullHotel`?

That way if, say, you want to change what you do with the information (say you want to send it as a text message later), you only have to change `hotel_database.rb` instead of replacing the `puts` in `NullHotel` AND in `Hotel`. We always want to minimize the amount of future code changes that we'll have to do.