[Rewritten] Slime Person Colors

Completely redid this code per suggestions of Gia and Carn (https://github.com/tgstation/-tg-station/pull/417)
No more blending icon operations, all possible slime icons are laid out in slimemutant.dmi
No more reliance on r_hair/b_hair/g_hair, slime_color has been added to track this in its place
---still relevent orignal notes---
This should help prevent xenobio from turning into a monotonous parade of blue identical looking slimefolk late round.
Taste the completely cosmetic rainbow!
http://i.imgur.com/WaK4EKn.png