python: represent message tags as unicode instances

Rather than returning simply strings and having to guess their encoding,
return explicit unicode() strings for the tags. Xapian stores UTF8, so
we know that they come as UTF8 encoded string.

Note: I tried to directly use the c_wchar_p type of the ctypes library
which translates directly into an unicode type, but that did not work
out so well, so we take c_char_p and .decode() them manually.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

bindings/python/notmuch/tag.py

@@ -23,10 +23,12 @@ from notmuch.globals import nmlib, STATUS, NotmuchError
 class Tags(object):
     """Represents a list of notmuch tags
 
-    This object provides an iterator over a list of notmuch tags. Do
-    note that the underlying library only provides a one-time iterator
-    (it cannot reset the iterator to the start). Thus iterating over
-    the function will "exhaust" the list of tags, and a subsequent
+    This object provides an iterator over a list of notmuch tags (which
+    are unicode instances). 
+
+    Do note that the underlying library only provides a one-time
+    iterator (it cannot reset the iterator to the start). Thus iterating
+    over the function will "exhaust" the list of tags, and a subsequent
     iteration attempt will raise a :exc:`NotmuchError`
     STATUS.NOT_INITIALIZED. Also note, that any function that uses
     iteration (nearly all) will also exhaust the tags. So both::
@@ -83,7 +85,7 @@ class Tags(object):
             raise NotmuchError(STATUS.NOT_INITIALIZED)
         # No need to call nmlib.notmuch_tags_valid(self._tags);
         # Tags._get safely returns None, if there is no more valid tag.
-        tag = Tags._get (self._tags)
+        tag = Tags._get(self._tags).decode('utf-8')
         if tag is None:
             self._tags = None
             raise StopIteration