rt-email-group-admin: Load users by email address after trying name

All of the examples in the usage use --user email@address, although the
individual subcommand usage does state that they are "unames".  While
many users in RT have a username equal to their email address, this is
non-obvious in the provided examples.  Simply make --user take a name or
an email address of an existing user.
rt-email-group-admin: Autocreate users if necessary

The command is much friendlier if you can specify any email address to
--user and have it autocreate a record for you.

I avoided using RT::User->LoadOrCreateByEmail to preserve preferred load
order (although it _shouldn't_ really matter) and use a correct Comment
rather than the misleading one in the existing method.
Merge branch '4.0/rt-email-group-load-user-by-email' into 4.0-trunk