Only traverse openers when on the top-level frame

• Avoid exceptions while broadcasting events

• Fixes an infinite recursion bug when window.opener === window

• Do not throw exceptions window.opener existed but has already closed.

• Do not throw exceptions when a frame.postMessage is denied.

• Exceptions are no longer thrown when publish, subscribe or unsubscribe were invoked directly.

    var publish = framebus.publish;
    publish('event');

• Breaking changes:
  • use of origin as a parameter for publish, subscribe and unsubscribe has been moved to the chaining-function target(origin)
• Added:
  • events can be published with multiple arguments. If the last argument is a function, it is treated as a callback that subscribers can call.
  • include(popup) adds popups to the listing of frames to be messaged.
  • window.opener will now be included in framebus messaging, if available.

Although we have been using this internally and in production for some time, this is our first public release!