* added -o switch for specifying output pathname for png image (perhaps we can get mat previews now! )
* hopefully fixed blak bug
* don't save image if size = 0
* improved wavelength sampling a little
* removed redundant BSDF calcs
* added fresnel_scale element to phong mat
* changed constant term on phong diffuse
* made bi-dir work with env-maps (still kinda sux tho)
* added max_num_consec_rejections to stuff that can be overriden in xml scene file.
* made network master print out time elapsed

The biggest new feature is the fresnel_scale element for the phong material.
What this does is provide a multiplier for the fraction of light reflected in the specular mode of reflection (as opposed to diffuse)
If you set it to 0, the Phong material will básically act as a diffuse material. The element is optional in the xml, and has default value of 1
which corresponds to the old phong behaviour. The value of the fresnel scale alos is the fraction of the incident light specularly reflected at grazing incidence. Hence it must be in the range [0, 1].