re: #119 - tidy up book reader view code

* _book-image.html.erb
  ** added logic that makes sure the attachment is an image before adding it to the bookImages array
  ** added a counter for the number of images added to the array
  ** removed some outdated code

* _display-media.html.erb
  ** brought back the html string builder so that </li><li> has no whitespace between it