don't crash with gateways with unknown message length

Crash "Can't have zero or negative length limit" occurred for
[INT]Odorik.cz gateway, because it had -1 as getSMSLength(). This should
hopefuly fix it.

https://code.google.com/p/esmska/issues/detail?id=463
always set problem detail when sending is unsuccessful

Hopefuly should fix:
https://code.google.com/p/esmska/issues/detail?id=464
set version to 1.7.99
improve user signature handling

Don't prepend it when not requested or do not double it. Basically
handle only multi-recipient mode.