Resolves #186

This adds a Rate class. It's a tool for running a loop at a constant rate.

Plus:

• Made context shutdown callbacks use weakref.WeakMethod().
  • Should prevent the callback list from growing infinitely.
• Added Handle.__bool__().
  • Used to avoid sleeping if the timer was destroyed.

Comparison with ROS 1:

• ROS 1: rate = rospy.Rate(10); ROS 2 rate = node.create_rate(10).
• Both raise ROSInterruptException if the client library has been shutdown
• ROS 1: may raise ROSTimeMovedBackwardsException; ROS 2: uses timer time jump logic.
  • If it went back more than 1 period, Rate sleeps for 1 full period. If it went backwards less than a period, Rate sleeps for the time remaining. If the time source changed, sleeps for the time remaining.
• ROS 1: Almost always ok to call in a callback; ROS 2: An executor must be serving the timer, so may block if called in a callback in a SingleThreadedExecutor
• ROS 1: Had Rate.remaining(); ROS 2: not implemented. It should be easy to add if someone needs it.