user routes changed. omniauth-kakao added.
added user_activation, structured users_controller simply.
added activation_ticket
email verify added
Complete kakao login cycle.
Need to rescue exceptions, to create views.
Merge with #153 (issue #147)
refactored with user_activator
send activation mail.
removed spec files not used
removed .DS_Store
added user show page, redirect bug fixed.
Complete user sign_in/sign_up logic
added model dependency
user_activator test added.
fix bug about issue #155
renewal admin/gate new, edit view
added api keys
Merge branch 'develop' into feat/auth-with-kakao

Conflicts:
	Gemfile
	app/controllers/application_controller.rb
	app/controllers/sessions_controller.rb
mail gem is changed to mailgun
bug fixed for invalid email (@user is nil)