Add status information to the API

[keimlink]

To decide if new emails have been received (see #11) or to display the number of new emails (see #5) the API must to provide this information. This should be done using a new status route. It should provide the total amount of emails received and also the number of unread emails.

The same information should be added to the domain route.

[keimlink]

Before the API can display the amount of unread emails #1 must be implemented first.