Added MJCF library files

VirtualRobot/CMakeLists.txt

VirtualRobot/MJCF/Document.cpp

VirtualRobot/MJCF/Document.h

VirtualRobot/MJCF/elements/Attribute.cpp

VirtualRobot/MJCF/elements/Attribute.h

VirtualRobot/MJCF/elements/Element.cpp

VirtualRobot/MJCF/elements/Element.h

VirtualRobot/MJCF/elements/actuator.cpp

VirtualRobot/MJCF/elements/actuator.h

VirtualRobot/MJCF/elements/asset.cpp

VirtualRobot/MJCF/elements/asset.h

VirtualRobot/MJCF/elements/body.cpp

VirtualRobot/MJCF/elements/body.h

VirtualRobot/MJCF/elements/compiler.cpp

VirtualRobot/MJCF/elements/compiler.h

VirtualRobot/MJCF/elements/contact.cpp

VirtualRobot/MJCF/elements/contact.h

VirtualRobot/MJCF/elements/custom.cpp

VirtualRobot/MJCF/elements/custom.h

VirtualRobot/MJCF/elements/default.cpp