- introduces a #wait method on the call operation view
- invokes #wait on a Notifier that is created for all operations
- ensures the Notifier is invoked if necessary whenever a client request
  completes
- updates the interop_test to use op.wait before checking if the call
  was cancelled.

- allow BadStatus to contain metadata that's populated by keyword args
- on servers, convert raised BadStatus metadata to trailers
- on clients, convert trailers to BadStatus metadata when raising BadStatus

- adds signal handling to RpcServer
- ensures that very little behaviour occurs in the signal handling context
- adds #run_till_terminated that launches a server in a thread and then handles
  signals that can be used to stop it.

- add rubocop as a dev dependency
- fixed many style violations it reported, often using --auto-correct
- add a rubocop config
 - .rubocop.yml shows the exceptions
 - .rubocopy_todo.yml tracks outstanding style issues
- adds a rake task to allow rubocop styling checks to be automated