added specular_power and shininess properties to the sphere record

added specular_power and shininess properties to the sphere record
added a specular_colour property to the point_light record
renamed the colour property of the point_light record to diffuse_colour
removed the diffuse_scale property from the point_light record
added a specular_term function to the lighting_function illumination model
added helper object_specular_power and object_shininess functions
updated the existing point_light in the scene with new properties
added an extra point_light to the scene to show specular lighting
updated the sphere objects in the scene with new properties
updated the scene test

git-svn-id: http://svn.plouj.com/repos/eraytracer@27 b8c14938-b733-429d-887c-d66ed1f8cd15