implemented reflections

implemented reflections
added more things TODO that I forgot about
added a reflectivity property to the material record
added a triangle shape record
added a recursion depth argument to raytraced_pixel_list, trace_ray_from_pixel and lighting_function
refactored the body of trace_ray_from_pixel into pixel_colour_from_ray function for which can be called recursively to a given depth
added a reflection term to the illumination model in lighting_function
added a triangle object case to ray_objec_intersect
added an epsilon to ray_sphere_intersect to deal with floating point inaccuracy and tracing reflected rays
stubbed ray_triangle_intersect
added a vector_reflect_about_normal function and test
added an object_reflectivity function
added reflectivity properties to the objects in the default scene
moved the blue sphere in the default sphere to make reflections on it more visible
added a triangle to the default scene
updated the scene test
set a default recursion depth to 3
commented out the tests that aren't supposed to pass yet

git-svn-id: http://svn.plouj.com/repos/eraytracer@31 b8c14938-b733-429d-887c-d66ed1f8cd15