Added Spline->SplineData boost::python::converter.

This allows Spline instances to be passed where Data is needed, in the same way as was already possible for the majority of SimpleTypedData types.
Merge branch 'master' of https://github.com/ImageEngine/cortex into coreGLMac
Merge branch 'master' into coreGLMac

Conflicts:
	glsl/IECoreGL/PointsPrimitive.h
	src/IECoreGL/IECoreGL.cpp
	src/IECoreGL/PointsPrimitive.cpp
	src/IECoreGL/Primitive.cpp
	src/IECoreGL/Shader.cpp
	test/IECoreGL/PointsPrimitive.py