Support cross-domain in both directions with single proxy page

[jcubic]

This is a continuation of failed PR #30 the code for this experiment is on the devel branch.

When this will be ready it should allow using sysend on CodePen for Cross-Domain communication.

[jcubic]

This demo should work: https://codepen.io/jcubic/pen/ZEvEVLG?editors=1010

[jcubic]

So the demo works but one way. I was testing the app and I'm almost sure that it's impossible to have both-way communication with a single proxy. You can only send from the page that doesn't have proxy to the one that has a proxy.

This is because to send to other domains you reach the iframe on the page. If there are no iframes for other domains you can't send the message.