Allow Answering Machine Detection (AMD) in Appointment Reminder

Currently, Appointment Reminder module will dial out and start playing the sequence of announcements, expecting key press from the called party for Confirm/Cancel/Reschedule etc.  It does it regardless whether the called party is human or an answering machine.  If the answering machine answers, and the module configured to make several calling attempts, the module will leave multiple identical voice messages at the receiving end.

This request is for an improvement to include answering machine recognition (? through Asterisk AMD()). 

Functionality-wise, the Appointment Reminder module may have configuration option to play a different announcement if the other end is an answering machine (e.g. just reminding of the appointment, without asking to press keys), or to hangup and try later X amount of times, as per reminder configuration. 

There can be an option to control whether calls must continue if the answering machine is detected and voice mail is left. A new status code may be introduced to areminder_calls (likely 5) to indicate that the system "left voicemail".