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.
IECoreNuke::LensDistort file parameter hash.
Changed the append() method to evaluate the file path before hashing it. This fixes a bug where TCL scripts in the file sequence knob were not getting evaluated before being hashed.
Added a required namespace specifier.
Merge pull request #64 from johnhaddon/gl2Compatibility

Replaced OpenGL 3 function calls with their 2.1 extensions
Merge pull request #65 from andrewkaufman/glLightingPreview

Maya GL Lighting Preview