function AJAX(config)
{

	if ( !(this instanceof AJAX) )
	{
		return new AJAX(config);
	}

	this._xhr = new XMLHttpRequest();
	this._config = this._extendOptions(config);

	this._assignEvents();

	this._beforeSend();
}

AJAX.prototype._extendOptions = function(config)
{
	var defaultConfig = JSON.parse(JSON.stringify(this._defaultConfig));

	for (var key in defaultConfig)
	{
		if (key in config)
		{
			continue;
		}
		config[key] = defaultConfig[key];

	}

	return config;
}

AJAX.prototype._assignEvents = function()
{
	this._xhr.addEventListener("readystatechange", this._handleResponse.bind(this), false);
	this._xhr.addEventListener("abort", this._handleError.bind(this), false);
	this._xhr.addEventListener("error", this._handleError.bind(this), false);
	this._xhr.addEventListener("timeout", this._handleError.bind(this), false);
};

AJAX.prototype._assignUserHeaders = function()
{
	if((Object.keys(this._config.headers)).length)
	{
		for (var key in this._config.headers)
		{
			this._xhr.setRequestHeader(key, this._config.headers[key]);
		}
	}
};

AJAX.prototype._open = function()
{
	this._xhr.open(
		this._config.type,
		this._config.url,
		this._config.options.async,
		this._config.options.username,
		this._config.options.password,
		);
	this._xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
	this._xhr.timeout = this._config.options.timeout;
};

AJAX.prototype._beforeSend = function()
{
	var isData = Object.keys(this._config.data).length >0,
		data = null;

		if(this._config.type.toUpperCase() === "POST" && isData)
		{
			data = this._serializeFormData(this._config.data);
		}	else
		if(this._config.type.toUpperCase() === "GET" && isData)
		{
			this._config.url += "?" + this._serializeData(this._config.data);
		}

	this._open();
	this._assignUserHeaders();
	this._send(data);
};

AJAX.prototype._send = function(data)
{
	this._xhr.send(data);
};

AJAX.prototype._handleResponse = function(e)
{
	if(this._xhr.readyState ===4 && this._xhr.status >= 200 && this._xhr.status < 400)
		{
			if(typeof this._config.success === "function")
			{
				this._config.success(this._xhr.response, this._xhr);
			}
		} else if (this._xhr.readyState === 4 && this._xhr.status >= 400)
			{
				this._handleError();
			}
};


AJAX.prototype._handleError = function(e)
{
	if(typeof this._config.failure === "function")
			{
				this._config.failure(this._xhr);
			}
};

AJAX.prototype._serializeData = function(data)
{
	var serialized = "";

	for (var key in data)
	{
		serialized += key + "=" + encodeURIComponent(data[key]) + "&";
	}

	return serialized.slice(0, serialized.length - 1);
};

AJAX.prototype._serializeFormData = function(data)
{
	var serialized = new FormData();

	for (var key in data)
	{
		serialized.append(key, data[key]);
	}

	return serialized;
}

AJAX.prototype._defaultConfig =
{
	type: "GET",
	url: window.location.href,
	data: {},
	options:
	{
		async: true,
		timeout: 0,
		username: null,
		password: null,
	},
	headers: {}
};

AJAX({
   type: "GET",
   url: "odbierz.php",
   data: {
       firstName: "Piotr ewe",
       lastName: "Kowalski Sas"
   },
   headers: {
       "X-My-Header": "123#asdf"
   },
   success: function(response, xhr) {
       console.log("Udało się! Status: " + xhr.status);
       console.log(response);
   },
   failure: function(xhr) {
       console.log("Wystąpił błąd. Status: " + xhr.status);
   }
});