Implement tagging using acts-as-taggable-on gem.

Following instructions from RailsCast #382.

* Install acts-as-taggable-on.
* Generate and run migration.
* Mark Resource#tag_list mass-assignable.
* Add tag_list input to Resource form fields.
* Add not_found_404 to ApplicationController.
* Add linkify_tags helper.
* Add match route for /tags.
* Create tags#show action/view.