Can get backdrops for movies

Add new class BackdropGrabber, change class name Grabber to PosterGrabber, add to module can_haz_poster self method that calls grab_backdrop method in BackdropGrabber class, also add integration test, and fixtures for it.

Usage: 
<% CanHazPoster.grab_backdrop('The Matrix', 1999).each do |bd|%>
  <%= bd %>
<% end %>