domain checking implementation

app/controllers/redmine_oauth_controller.rb

app/views/hooks/_view_account_login_bottom.html.erb

app/views/settings/_google_settings.html.erb

assets/images/google_login_icon.jpg

assets/stylesheets/buttons.css

config/locales/en.yml

config/locales/ru.yml

init.rb

lib/helpers/checker.rb

lib/helpers/mail_helper.rb

test/functional/redmine_oauth_controller_test.rb