TapQueue server and station 0.15.0

Added

• Email notifications. The server emails the people you list when a printer is unreachable or
  stops (paper jam, out of paper), when a release station stops checking in, and when a PC's
  client crashes, and again when a printer or station recovers. Low toner and other warnings can
  be added. The emails go out through your own mail server or provider: enter its SMTP settings
  on the console's new Notifications page (STARTTLS, TLS, or neither for a local relay), send
  a test email, and turn them on. A printer or station has to stay wrong for 5 minutes first
  (changeable), a PC that keeps crashing sends at most one email an hour, and an email that can't
  be sent is tried again for a day. tapqueue-admin notifications shows the settings, sends a
  test and turns them on or off. See docs/notifications.md.