Pointing to new cortex site
Added code for building LLVM and OSL dependencies.
Carved out a TypeId range for GafferOSL.

The range is taken from GafferRenderMan.
Added a todo in ImageProcessor.

The hashing doesn't seem quite right.
Moved ArnoldShader::parameterValue() implementation to base class.

The base class implementation makes sense alongside the base class implementation of parameterHash(), and should work for any shader types which support the linking of parameters by specifying a value of "link:shaderHandle" to the renderer. It paves the way for an easier implementation of the upcoming OSLShader.

The RenderManShader implementation of parameterValue() still isn't shared by the base class but hopefully one day it can be - at least we'll be moving in the right direction by sharing code between ArnoldShader and OSLShader.
Stopped annoying startup errors when 3delight or arnold are missing.

We now only attempt to load GafferArnold if the arnold python module is on the PYTHONPATH, and only attempt to load GafferRenderMan if the DELIGHT environment variable is set. We do still catch and report errors if loading subsequently fails due to a configuration error though. Since the conditions under which we try to load the modules are created by the steps in the getting started guide, hopefully this will work reasonably well, not making false attempts at loading and still providing hints when things aren't quite right.
Added GafferOSL module with OSLShader node and test cases.

This is the first step in supporting Open Shading Language in Gaffer. Upcoming commits will add the ability to actually execute the shaders to create images.