Request: Implement Class.Thenable. Fixes #2643.

Thenable example:

	var request = new Request();
	request.send().then(function(value){}, function(reason){});

Example, hooking into an actual Promise:

	var promise = new Promise(function(resolve, reject){
		var request = new Request();
		request.send().then(resolve, reject);
	});
	promise.then(function(value){}, function(reason){});