Removed Unnecessary Functions

Combined SendSMS(), StartSMS() and EndSMS(). Removed related variables
and accessor functions.
Added a ReceiveSMS Callback Function

The function onReceiveSMS will be executed when a SMS message is
received. Message checking is done in onRead() to ensure message is not
lost.
Added Delay After Hangup

A delay allows the module to return to 'normal'. Calling any other
methods (especially SendSMS) can result in a CMS 313 Error.