Log the wait before sending email

When waiting before sending email, log the wait. Fixes #1195173 [Feature Request - Multiple books emailed to device](https://bugs.launchpad.net/calibre/+bug/1195173)

src/calibre/gui2/email.py

@@ -92,7 +92,11 @@ class Sendmail(object):
                 raise worker.exception
 
     def sendmail(self, attachment, aname, to, subject, text, log):
+        logged = False
         while time.time() - self.last_send_time <= self.rate_limit:
+            if not logged:
+                log('Waiting %s seconds before sending, to avoid being marked as spam.\nYou can control this delay via Preferences->Tweaks' % self.rate_limit)
+                logged = True
             time.sleep(1)
         try:
             opts = email_config().parse()