Implement more nodes

[vkoskiv]

Here's a big list to track where we're at. Progress is currently happening in the node-improvements branch.
A lot of functionality is still written in the core logic that could be moved over to using nodes instead. I'll mark nodes that require refactoring with a *
For the most part, I'm looking at the nodes available in the Cycles rendering engine to spot some easy nodes to start with.

• texture node (TODO: Rename to color, perhaps)
• Diffuse BSDF
• Glass BSDF
• Metal BSDF
• (Dubious) Plastic BSDF (combines diffuse + glass)
• Mix node
• Grayscale converter node
• Texture coordinate node (51ce446)
• Fresnel node *
• Transparent BSDF (Alpha blending) * (dda5c92)
• Emission bsdf * (sort of implemented in f4d52e3 in a hacky way)
• Sky texture (Logic exists in sky.c, need to expose as a node)
• Environment map as an output node.
• Add shader node
• blackbody node
• Value type
• Vector type
• Math node
• Vecmath node