added new computed property lastUpdatedISO to controllers/superheroes/show,
  returns new lastUpdated field in ISO format

added vendor/jquery.timeago.js to index.html

added new lastUpdated field to superhero model - added default
  value to fixtures

added new _setLastUpdated observer to superhero model,
  automatically updates lastUpdated when a property changes

invoke view date-timeago in _show_top_row.hbs

new view and template date-timeago - inserts current time and
  invokes jquery.timeago

new jQuery module jquery.timeago.js - rewrites timestamps as
  "about 6 minutes ago" style
Merge pull request #39 from trianglegrrl/last-updated

Last updated field in superhero model