Rebuild JavaScript events API

[michalstocki]

It would be nice to rebuild event's interface. We would like to replace one callback with different parameters to JavaScript-specific

FWRecorder.addEventListener("eventName", function(){
    // ...
});

interface. This will allow avoid using large case statement.

Another option is to use JS signals: https://github.com/millermedeiros/js-signals

Yet another option is to use a "jQuery style" events lisening:

FWRecorder.on("eventName", ["sessionName"], function() {
    // ...
});