* PEP8 cleanup
  - code simplifications
  - avoid empty except
  - sort imports
  - format line breaks
* Code cleanup
  - Removed deprecated options --legacy, --inorder, --check-order, --includes
  - Require all xacro commands to be prefixed with 'xacro:'
  - Added missing copyright notices
  - Removed python2 stuff
* Adapt Travis config to use colcon
* Provide ROS2-based replacement for substition args
* Modified package structure as per ROS2 rules
  - Using ament_cmake
  - Moved source contents from "src/xacro" to "xacro"
  - Modified package.xml and setup.py according to ROS2 requirements
  - Configured completion hook
  - Removed catkin artifacts from cmake extension
* Contributors: vandanamandlik, Jacob Perron, Robert Haschke