Add user invitation

https://trello.com/c/5tVdI7wQ/14-create-user-by-organization-admin-and-add-him-to-organization
@psmir you should try to search existing user by email and add him if he exists.

You may leave full name blank (change validation that for invited user full name is not presence).

For password generate new token (devise has a function to generate secure token or you may use SecureRandom class from ruby)