Added spider meat, meals, and butchering

Fixes #1511

* Adds spider meat, spider legs and spider eggs
* Adds spidermeat meatloaf, boiled spider leg, carp sashimi and green eggs and ham
* Added butchery system for spiders where their legs can be hacked off one by one with a 50% chance to successfully harvest a leg, and updated icons, until the final carcass drops spider meat (eggs from nurses)
* Spider sprites updated to have eight legs rather than six visible
* Refactored simple_animals.dm harvesting slightly, still needs istype() check for kitchen and butcher's knife removed (SHARP flags please)

![](http://i.imgur.com/p5A0813.png)

Sprites by Iatots