Add mongoid-paperclip plugin and generate Image model

Conflicts:
	config/routes.rb
Add image uploading function

Conflicts:
	app/views/contests/problems/_form.html.erb
	config/routes.rb
Remove unused file
Merge branch 'master' into 23_image_uploader

Conflicts:
	app/views/contests/problems/_form.html.erb
Merge pull request #50 from TakenokoChocoHolic/23_image_uploader

23 image uploader