RFC - Asynchronous client requests in Tracy (AJAX)

Pull request for [this RFC](http://forum.nette.org/en/19724-asynchronous-client-requests-in-tracy-ajax),

Discuss on forum, code comments here, please.
Ok, I got pretty much confused by that dev/ folder, as it is RFC code which shouldn't have such things (those should be in another branch/repo). :smiley_cat: