Add support for multiple emails for each user

Now the match between Trac users and GitHub users is based on emails
    for every user with a valid login ("logins" in config file).
    Note: the first "logins" entry is skipped since is the fallback
          user account.
Every time an e-mail from Trac db doesn't match any valid users (or
    it's not possible to extract the e-mail for a user from the
    Trac db), a string like "Original Author: <user>" is added in the
    issue's body as header. (In such case the fallback account is used)
"users" configuration entry is currently ignored.