Indigo_05 test07

Changelog:
* added -o switch for specifying output pathname for (*.png) image (perhaps we can get (*.mat) previews now.)
* hopefully fixed black bug.
* dont save image if size = 0
* improved wavelength sampling a Little.
* removed redundant bsdf calcs.
* added fresnel_scale element todo Phong (*.mat)
* changed constant term on Phong Diffuse.
* made bi-dir work with env-maps (still kinda sux tho)
* added max_num_consec_rejections todo Stuff that can be overriden in xml scene file.
* made network máster print out time elapsed.

The biggest new característica 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 todo Diffuse)
If you set it todo 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 todo the old Phong behaviour. The value of the Fresnel scale a los is the fraction of the incident light specularly reflected at grazing incidence. Hence it must be in the range [0, 1]. http://www.bennolan.com/nick/indigo_05_test7.zip.