Making extension fully browser-agnostic

Also added some doc too
Extension related changes

We changed extension.js in a way that changes had to be made here too.

Hoping for that not happening anymore.
Chrome adaptations

Now, chrome.js implements an proxy to xhr calls.

Please, refer to in-file comments to a more verbose, friendly and
detailed explanation.
Merge branch 'feature-PHPShell-event-listeners' into develop

Resolves #9