libravatar: Encode email address before hashing

With Python 3, the libravatar plugin yields the following error:

   Unicode-objects must be encoded before hashing

This is caused by the fact that the email string obtained from the
header of the content file is stored as a Unicode object in the
metadata list in Python 3.  The string must then be encoded before
being passed to the hashlib.md5 function.  According to the current
standard, only ASCII characters should appear in email addresses, so
encoding metadata['email'] into ASCII will make the plugin work with
both Python 2 and Python 3.

libravatar/libravatar.py

@@ -42,7 +42,9 @@ def add_libravatar (generator, metadata):
     if metadata ['email']:
 
         ## Compose URL using the MD5 hash
-        md5 = hashlib.md5 (metadata ['email'].lower ()).hexdigest ()
+        ## (the ascii encoding is necessary for Python3)
+        email = metadata ['email'].lower ().encode ('ascii')
+        md5 = hashlib.md5 (email).hexdigest ()
         url = 'http://cdn.libravatar.org/avatar/' + md5
 
         ## Add eventual "missing picture" option