Set In-Reply-To mail header for check followups

This allows mail clients to know to what message the followup is for,
making it easy for an user to see what relates to what when there are
several checks.

lib/mails.py

 import smtplib
 from email.mime.text import MIMEText
+from email.utils import make_msgid
 from collections import defaultdict
 from sys import stderr
 from time import strftime
@@ -88,6 +89,14 @@ def send_email_for_check(check):
     # encode / decode is a fix that didn't make it into Debian Wheezy
     # http://bugs.python.org/issue16948
     msg = MIMEText(msg_text.encode('utf-8').decode('latin1'), 'plain', 'utf-8')
+
+    msg['Message-ID'] = make_msgid(type(check).__name__)
+    # if check is OK it's a follow up, so set In-Reply-To
+    if check.ok and hasattr(check, 'mails_msgid'):
+        msg['In-Reply-To'] = check.mails_msgid
+        msg['References'] = check.mails_msgid
+    check.mails_msgid = msg['Message-ID']
+
     msg['Subject'] = subject
     msg['From']    = config.emails.addr_from
     msg['To']      = ", ".join(config.emails.to)