Make serialized stanza as a optional choice.

The serialized stanza is not working with Facebook XMPP server when dealing
with UTF-8 of chinese, korean, and etc. It is better to make it as optional
choice instead of hard-coded.

Signed-off-by: Jia-He Jou <joujiahe@gmail.com>
Merge pull request #34 from joujiahe/master

Make serialized stanza as a optional choice.