ENH: Modernize RIBExporter and RIBProperty
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.