Fix split normal message

Implemented fix mentioned in this issue:

https://github.com/futomi/node-sms-pdu/issues/5

Simple normal long sms without any special chars were giving this error:

RangeError [ERR_OUT_OF_RANGE]: The value of "offset" is out of range. It must be >= 0 and <= 47. Received 4

First release

0.0.1

Add validity period