This introduces a hierarchy of classes implementing
* ETS TS 102 225 (general command structure)
* 3GPP TS 31.115 (dialects for SMS-PP)
In this initial patch only the SMS "dialect" is supported,
but it is foreseen that USSD/SMSCB/HTTPS dialects can be
added at a later point.
Change-Id: I193ff4712c8503279c017b4b1324f0c3d38b9f84