Some utf-8 <-> unicode problems

[Jajcus]

• A few classes in jabber.dataforms require unicode arguments (no check done, only documentation), but fail to convert them to utf-8 encoded strings before passing to libxml2 methods. This causes UnicodeEncodeError exceptions.
• stanza.set_content() and stanza.add_content() don't accept unicode objects as their arguments

The following patch fixes these issues:
http://kolos.math.uni.lodz.pl/~grzywacz/pyxmpp_unicode.patch

[Jajcus]

--jajcus

[Jajcus]

(In [709]) - unicode handling improvments by 'grzyw' (closes #15)

--jajcus