Implement actions API for use by client libraries.

Actions are a important feature/concept in ROS that allow for tracking and updates to long-running requests called "goals". Refer to the design doc for more information: ros2/design#193

Prerequisites:

• C typesupport for actions (rosidl_generator_c) Generate C typesupport for Actions rosidl#302

Tasks:

• Propose header files for action server, action client, and other relevant entities Add rcl_action package and headers #307
• Architecture doc
• Implementation:
  • Goal state machine Action goal state machine tests #311
  • Types implementation [rcl_action] Implement init/fini functions for types #312
  • Goal handle implementation [rcl_action] Implement goal handle #320
  • Action client implementation Action client implementation #319
  • Action server implementation Action server implementation #323
  • Wait set groups Wait set groups #308
    • Integrate with rcl_action [rcl_action] Implement wait set functions for action server #337

connects to ros2/ros2#583