1. Add iframe_html to course
2. Fix N+1 query in CoursesController
	modified:   Gemfile
	modified:   Gemfile.lock
	modified:   app/controllers/admin/courses_controller.rb
	modified:   app/controllers/courses_controller.rb
	modified:   app/views/admin/courses/_form.html.erb
	modified:   app/views/courses/show.html.erb
	modified:   app/views/stages/_stage.html.erb
	new file:   db/migrate/20140822174137_add_register_iframe_html_to_courses.rb
	modified:   db/schema.rb