Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
Per my comment in https://github.com/strongloop/loopback-workspace/pull/164, you should read the default context options via `app.get('remoting')`. It means you need to move the code building `contextHandler` to `restApiHandler`.

If you want to make this change in a new pull request (or leave it up to me to make it), then I am fine with that too.