Observers JS Module Improvements

I've updated the Observers JS module with a number of changes. In particular, it's now possible to specify a this object when adding an observer that is a method, so instead of doing this:

let MyObserver = {
  onFoo: function(subject) {...},
  onBar: function(subject) {...},
  onBaz: function(subject) {...},
  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
  observe: function(subject, topic, data) {
    switch(topic) {
      case "foo":
        this.onFoo(subject);
        break;
      case "bar":
        this.onBar(subject);
        break;
      case "baz":
        this.onBaz(subject);
        break;
    }
  }
}

Observers.add("foo", MyObserver);
Observers.add("bar", MyObserver);
Observers.add("baz", MyObserver);

You can do this:

let MyObserver = {
  onFoo: function(subject) {...},
  onBar: function(subject) {...},
  onBaz: function(subject) {...}
}

Observers.add("foo", MyObserver.onFoo, MyObserver);
Observers.add("bar", MyObserver.onBar, MyObserver);
Observers.add("baz", MyObserver.onBaz, MyObserver);

Other changes include a couple of bug fixes and some backwards-incompatible API improvements. See the documentation for all the details (including steps to update from the previous version of the module).