Adds directional blood trails

The blood trails now follow the path the body has taken and players can
very easily make out said path.
Heavily damaged corpses will have a much thicker blood trail than
corpses with some brute damage. At the moment, trails overwrite
eachother and a placeholder icon set is used for alien heavy blood
trails.
Makes Supply Pack datums no longer garbage collect. Prevents them from making artifacts like mulebot #3. Fixes issue #137.

Moves the supply shuttle controller to the controller folder and adds its creation to the new proc of the master controller.
Also, makes the supply shuttle make supply packs on new instead process.
Merge branch 'master' of https://github.com/tgstation/-tg-station into Supplypacks
Black slime extract now inflicts a disease that turns the victim into a slime after a short while instead of immediately turning them through the chem itself. This gives people a chance to call out over radio about what happened if they were the victim of a drive-by sliming. Likewise it gives them a faint hope of reversing the process with the cure, frost oil (in the code frost oil is hilariously toxic to slimes). Source of frost oil are botany and (irony) xenobio. The disease is fast spreading, and infected are likely to only have 30 seconds to a minute to find frost oil, but it's highly effective once taken.

On the plus side for xenobiologists this allows for them to place black slime extract in things (medibots, food) without it becoming immediately apparent to victims what's been sabotaged.

This disease cannot be spread in any fashion besides black slime extract. (Because that would be so hideously broken if it was transmittable)

The flavor texts for the stages are an allusion to nethack, where the process of getting slimed is much the same (http://nethack.wikia.com/wiki/Sliming)

One other feature of note: mutant slime people retain the old black slime extract behavior, getting slimed almost immediately after exposure. This means that you can replicate pre-pull black extract by creating a mixture of equal parts black and green extract.
Standardized transformation diseases
forgot to update the disease paths for their respective reagents
runtime fix:  pick() from empty list
Added a strategic ismob so that if for some reason new_mob is something that isn't a mob it doesn't runtime.

BADMIN'S NOTE: This means you can var edit a tranformation virus' new_form to change what an infectee will turn into pretty much anything. Turning into something that isn't a mob will ghostize the player obviously.
blood trails now stack

--DO NOT MERGE STILL WIP--
Makes blood trails use overlays, the overlays are contained in a holder
object. DNA is now properly added to them.

todo: redo light, xeno light and xeno heavy trails. help of spriters is
welcome.
Restoring more up to date versions of robotic_transformation.dm and xeno_transformation.dm
only to delete them again in a flurry of destruction!
Updated Chemistry-Reagents.dm to hopefully resolve the conflict
Merge branch 'master' of https://github.com/tgstation/-tg-station into sliming

Conflicts:
	code/datums/diseases/robotic_transformation.dm
	code/datums/diseases/xeno_transformation.dm
	code/modules/reagents/Chemistry-Reagents.dm
Moves some datacore code into datacore.dm from other .dm's
AI Photography, with extensions
AI Photography, with extensions
Merge branch 'AIphotography' of https://github.com/AndroidSFV/-tg-station into AIphotography
Removes bad WIP I had commented out
Changelog
Uncommented pixel_x and pixel_y