New combined TEXT/PDU mode gsmmodem:send_sms blowing up on max_messages argument

[jwishnie]

The combined merge blew away support for this as an oversight.

[jwishnie]

These two commits fix:
http://github.com/rapidsms/pygsm/commit/981dfa90ab9c0d978428091377e1586d3b8f1ef1
http://github.com/rapidsms/pygsm/commit/f84f5bae5675193fd75bdc31cfc45bfa11e41d3c

[czue]

Looks good, thanks! If it all works go ahead and push. I like the doc to code ratio....

[jwishnie]

Added some error checking on max_message arg and some unit tests:
http://github.com/rapidsms/pygsm/commit/32c46bd153d632232be1e3afd5c067ec234654d9
http://github.com/rapidsms/pygsm/commit/9b7a33b41f889a20ba336c6fa050850f102018bb

[czue]

Cool looks good (again if it works). Also this doesn't quite make the cut, but remember that commits that are only docs and tests don't need review.