ChaptersHelper tagging and category search capabilities

This branch adds tag functionality methods to the ChaptersHelper, using the ActsAsTaggableOn gem.

It adds the ActsAsTaggableOn gem to the Gemfile, adds acts_as_taggable to the Chapter model, adds four tag methods and a category method to the ChaptersHelper file, and finally requires chapters_helper.rb / includes the ChapterHelper in the ChaptersController.