[WIP] added connected callback

[ChrisBAshton]

Opened a PR from my pym.js fork of this one.

DO NOT MERGE yet - await the outcome of https://github.com/bbc/news-vj-developer-scaffold-iframe-wrapper/pull/30.

Pym.js

BBC-specific modifications

• We added the ability to init a pym Parent with a sourcedoc attribute rather than an iFrame URL.
• We added a way of accessing host information:

1. When creating pym Parent, pass a config property with whatever you like (e.g. host URL, cookies, etc)
2. When creating pym Child, pass a connectedCallback function, which is invoked once a connection has been made with the pym Parent.
3. Anything inside the callback can safely call pym Child.getHostInformation() - this provides a nice easy way for pym Childs to access host information.

Example:

var myConf = {
    hostInformation: {
        onBbcDomain: true,
        category:    'news'
    },
    applicationHtml: coreContentContainer.innerHTML
};
var pymParent = new pym.Parent(iframeUid, iframeUrl, {config: myConf});

...and the child:

new pym.Child({
    polling: 100,
    connectedCallback: function downloadApplication(self) {
        console.log(self.getHostInformation().hostInformation);
    }
});