Check SSL certificate for expiration

We currently don't care about expiration dates of the servers SSL
certificate. This patch adds a check that fails Cert verification when
it is past its due date. There is no way or option to override this
check.

Unfortunately we only seem to be able to get SSL certificate data when
we passed in a CA cert file? How do we get that date when we don't have
a ca cert file?

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaplibutil.py

@@ -229,6 +229,13 @@ class WrappedIMAP4_SSL(UsefulIMAPMixIn, IMAP4_SSL):
         dnsname = hostname.lower()
         certnames = []
 
+        # cert expired?
+        notafter = cert.get('notAfter') 
+        if notafter:
+            if time.time() >= ssl.cert_time_to_seconds(notafter):
+                return ('server certificate error: certificate expired %s'
+                        ) % notafter
+
         # First read commonName
         for s in cert.get('subject', []):
             key, value = s[0]