Add alternative email addresses to the model

A new struct is created named EmailAddress that contains alternative
email addresses for users. Also the email related methods; IsEmailUsed
and GetUserByEmail are updated.

DeleteUser deletes the extra email addresses and DeleteInactivateUsers
also deletes inactive accounts. This could be factored out, but should
do it for now.
Add function to the model for email address management (add/delete/activate)
Method for activating email addresses through verification email
Template for email activation email
Finish method for having multiple emails/user.

All basics are implemented. Missing are the right (localized) strings
and the page markup could have a look at by a frontend guy.
Small fixes to multiple_emails feature
Merge pull request #755 from phsmit/multiple_emails

Multiple emails