XEP-0297 Stanza Forwarding implementation

This patch adds Forwarded.java, a class to wrap messages forwarded
from a different entity. A forwarded stanza contains of a Packet and
an optional timestamp.

WARNING: The current implementation only allows to forward Message
packets, as there is no universal way to parse a Packet from SMACK.

Signed-Off-By: Georg Lukas <georg@op-co.de>