Can return backdrops for movie

Change previous class `Grabber` to `PosterGrabber`, add class `BackdropGrabber` that returns backgrounds for movie, add in module `can_haz_poster` self method `grab_backdrop` wich will call class method and return the backdrops for movie also add integration test on class `BackdropGrabber` and fixtures for it

<% CanHazPoster.grab_backdrop('The Matrix', 1999).each do |bg| %>
  <%= bg %>
<% end %>