Developer portal strings localization

A customer is asking for the dutch localization of these strings, it's possible that the customer will ask for further strings to be localized.

"Signed in successfully" found -->
https://github.com/3scale/porta/blob/master/lib/developer_portal/app/controllers/developer_portal/login_controller.rb#L32

"Incorrect email or password. Please try again." found -->
https://github.com/3scale/porta/blob/master/app/lib/authentication/strategy/internal.rb#L11

"You have been logged out" found -->
https://github.com/3scale/porta/blob/master/lib/developer_portal/app/controllers/developer_portal/login_controller.rb#L46