join SMS fragments in history

When a message is split into several fragments (to be sent separately),
they are no longer visible as separate pieces in the history (which
makes it hard to forward the message), but they are instead visible as
the original single message.

Fixes: https://code.google.com/p/esmska/issues/detail?id=185
Contributed by: Lenka Saidlova <lenkasaidlova@seznam.cz>
Pull request: https://github.com/kparal/esmska/pull/10