Changed Ward isotropic normal input from "vector" to "normal" type. Changed default eta on Fresnel block from 1.5 to relative index of refraction (to the air ior of 1.0), so eta defaults to 0.75, good default for dielectric surfaces.
Added thin film interference example network (uses 'uffizi-large.tdl' environment map, from Paul Debevec's HDR gallery, http://www.debevec.org ).
