Fixed URDF materials parsing and application (of transparent materials). Added possibility to set up ViewBuilder inside plugin and to visually debug physics.

Fixed robot's materials parsing (when using urdf geometry, nmat wasn't updated as it should, fixed) and setting up (enabling transparency, previously alpha value was discarded, all materials were opaque).
Changed SimulatedDevice signature a bit to enable additional setup in ViewBuilder (new setViewBuilder method and made applyPhysics method optional to implement).
Enable physics debugging using --debugPhysics flag with optional numeric parameter.