Corrige un bug lors de la génération des index

Les characters spéciaux (?, $, €, ...) faisaient planter la compilation

patacrep/index.py

@@ -73,7 +73,11 @@ class Index(object):
     @staticmethod
     def get_first_letter(key):
         """Return the uppercase first letter of key."""
+        try:
             letter = FIRST_LETTER_PATTERN.match(key).group(1)
+        except AttributeError:
+            # classify as number all the non letter characters 
+            letter = "0"
         if re.match(ur'\d', letter):
             letter = '0-9'
         return letter.upper()