Add Shader handles

[ikeycode]

We need a nice cache for Vertex and Fragment shaders, with double lookups (int key or slow string find)

This will allow us to cache our builtin shaders, as well as allowing the consumer to completely override the shaders. Eventually we'll also want a way to specify custom shader rules for various parts of the codebase,such as in the tiled renderer. i.e. it'd be nice to use a shader to make the
trees 'move', etc.

We'll also want this to be not-ugly, so lets use structs, not classes, and have comparable handles. This will allow us in future to sort through batches if the underlying shader or texture changes. (Read: Material)