allow user to invite again

based on configured number of days between invites so its not deemed
spamming
fix sort code
cleanup dups check
fix tests and invite window code
simplify days ago by using time.Now().AddDate()
Merge pull request #10 from tidepool-org/jhbate/allow-multi-invites

Jhbate/allow multi invites
Merge branch 'master' into jhbate/pw-recovery