Added limited support for pairing

Multiple users can be selected at once, with some caveats:
- combined email prefix is 'dev' (will be configurable)
- email domain is the domain of the first requested address (will be
  configurable)
- the same name and address can be given multiple times (will be
  restricted)
Updated TODOs
Fixed bug for when user not found

When selecting multiple users, if one wasn't found, an error was raised
Sort users by email when selecting multiple users

When selecting multiple users, the combined user is now sorted
alphabetically, by email address
Minor cleanup

Cleaned up spec
Disallow multiple selection of same user

If the same user is entered twice (or found twice) it's not included
again in the combined user
Prioritised features for next release
Refactored user-combining

Moved user-combining code into user class
Tightened method visibility