Added Save trim to Disarm Stick 15Sec

PLEASE NOTE: First Pull request from me ever.

Added savetrim_manual flag to AP_Notify.h and Buzzer.h
Added if in Buzzer.cpp
Added SAVETRIM_DELAY to motors.pde
increased arming(disarm) counter to SAVETRIM_DELAY +1
set AP_Notify::flags.savetrim_manual = true; after save trim
Reset AP_Notify::flags.savetrim_manual = false; on yaw center

I will update manual after request has been approved but please update release notes for me.