The original RIB classes were created to work with BMRT, a free
Renderman implementation written by Larry Gritz.

This patch updates the RIB classes to conform to the latest Renderman
RIB syntax. Motivated by the Pixar release of a no cost Renderman:
http://renderman.pixar.com/view/renderman

Adds two new tests:

TestRIBExporter generates a valid rib file that should work with
Renderman. This is meant to be a regression test for a Renderman
rendered image.

UnitTestRIB attempts to cover all of the RIB code including errors and
warnings.

If Renderman is detected on the system, the RIB file generated by
TestRIBExporter is processed and the resulting image is checked for
changes from a baseline.