Replacing Helper#display_title with Model#to_s

By using #to_s we can consistently render an object in any context,
instead of sending it through a transformation function. This also means
that the "display_title" for an object can be accessed in any context.
Merge pull request #106 from jeremyf/replacing-display-title-with-to_s

Replacing Helper#display_title with Model#to_s