Add NPC Greetings

This addresses #2134 and enables NPCs to display a customizable greeting the first time the player talks to the NPC. 
https://cloud.githubusercontent.com/assets/2179547/3608544/e9df93f2-0d62-11e4-86f3-6e23c3493199.png
 I would like to add tasty text color support to this which involves adding tasty text support to the dialogue.  I'm not quite sure how to go about that.  Maybe @niamu could help out?