[hextilt] Added hextilt (quadrotor with 5 dofs arm)

python/example_robot_data/robots_loader.py

robots/hextilt_description/README.md

robots/hextilt_description/meshes/base_link.dae

robots/hextilt_description/meshes/hextilt.dae

robots/hextilt_description/meshes/link_1.dae

robots/hextilt_description/meshes/link_2.dae

robots/hextilt_description/meshes/link_3.dae

robots/hextilt_description/meshes/link_4.dae

robots/hextilt_description/meshes/link_5.dae

robots/hextilt_description/urdf/hextilt_flying_arm_5.urdf