Adding IECoreGL::Shader::lambertFragmentSource.

Also passing fragmentP through in defaultVertexSource, adding a no-op defaultGeometrySource, and a constantFragmentSource.
Using LightingMode in IECoreMaya DisplayState

DisplayState::baseState now accepts an optional parameter for lighting mode. If the value is not kLightDefault,
then the lambert shader from IECoreGL will be used to override the ShaderStateComponent.
Merge pull request #65 from andrewkaufman/glLightingPreview

Maya GL Lighting Preview