Support for async request-respond through post-message

[sslash]

There are cases when banners need to communicate with the parent, and be notified when there is a response.

Example use:

// host-plugin
pluginApi.reqres.respondTo('some:ext-event', function(evt, done) {
    // do stuff with evt.data.foo
    done(someValueToBePassedDown);
});

// ext-plugin
pluginApi.reqres.request(window, 'some:ext-event', {foo:bar}, function(evt) {
    // do something with the value that was passed down through evt.data
});

[sveisvei]

reqres seams non-intuitive namespace, maybe choose a better name (transport/messageBus etc?) - or remove the namespace.

Can the ext plugin pass anything else than window to the request function? I see no reason for this first arg.

[sslash]

Naming it a message bus is a bit overkill imo, in adherence to the messaging design pattern. The pluginAPI itself is a simple pub/sub system. This service however is more specialized and conforms to the request/response pattern (with an asynchronous response handler). I think that should be expressed in the API name. But yes, reqres might not be very intuitive. Maybe I shouldn't have abstracted out these two functions, but instead have them directly on the pluginApi. What do you think?

request needs a window reference, because it depends on xde, which uses for post messaging.

[sveisvei]

// host-plugin
pluginApi.respondToPostmessage('endPoint', function(payload, done) {
    //data
    done(Math.random());
});

// ext-plugin
pluginApi.postMessage('endPoint', {'foo': bar}, function(err, result) {
    // error is e.g. reference error
    // do something with the value that was passed down through evt.data
}, window);