Tortoise: add neighbors4 prim
Tortoise: add first/last/butfirst/butlast/lput/fput prims
Tortoise: add no-turtles/no-patches prims
Tortoise: add patch-set prim
Tortoise: add abs prim
Tortoise: add (modified) Sandpile model to docking tests

.nlogo file for Sandpile lists the differences from the original model
(mouse support, etc)
Merge branch 'tortoise' into tortoise-sandpile
Merge pull request #457 from NetLogo/tortoise-sandpile

Tortoise: yay, Sandpile working