python: represent message tags as unicode instances

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>