Enable explicit setting of postMessage targetOrigin param.
Refactoring calls postMessage into a method.
Refactoring instantiation.
Makes more sense for this to be a method on the communicator, rather
than a sendable that has to be dynmically created on the sender.
Removing communicator setter method.
Setter is unnecessarily clunky. Obviating the need for this method
means there's one less thing you need to do to integrate with this
library.
Only create senders on instantiation if there are any.