Add support for Spine2D animations

[bsergent]

This should make animations more smooth and fluid, while also allowing for customized clothing/hair/coloring of the player, and should make the animation workflow quicker.

Provides solution for #36 and #37. Deprecates #49 since Aseprite layers would be used for building Spine animation sheets instead. For the hidden collision layer idea, I could instead for none-clear pixels. Probably stick with collision circles though.

References:
Spine2D