Spawn Gunther's Visage (npc 5666) at end of quest 411

Script to run after completing quest 411 The Prodigal Lich Returns. C++ code by @kldxzzhc2 in  #12482
Modification: Added actual NPC text from http://www.wowwiki.com/Gunther_Arcanus .
Please initialize all fields in the constructor too. you can take a look at my recent commits which call the same Initialize() function in both constructor and Reset() to avoid duplicate code