re: #119 - tidy up book reader view code

* _book-image.html.erb
  ** jslint’ed the js
  ** added jsdoc comments
  ** corrected an issue where the bookImages.join() was outputting an comma when there was only one image in the array
  ** made the br functions a bit more robust so they can handle a case where, for whatever reason, the bookImages array is not properly created

* _display-media.html.erb
  ** added logic so that the page mode passed in the book reader url will tell it to display only one page for a single image and two pages for anything > 1 image