实现兼容document.querySelector的方法

var querySelector = function(selector) { //TODO 先简单兼容，后续继续扩展；
	var element = null;
	if(document.querySelector) {
		element = document.querySelector(selector);
	} else {
		var selectors = selector.split(' ');
		var context = document;
		for(var i = 0, len = selectors.length; i < len; i++) {
			context = handleQuery(selectors[i], context);
		}
		element = context;
	}
	return element;
};
var handleQuery = function(selector, context) {
	var idSelectorRE = /^#([\w-]+)$/;
	var classSelectorRE = /^\.([\w-]+)$/;
	var tagSelectorRE = /^[\w-]+$/;
	var element = null;
	if(idSelectorRE.test(selector)) {
		element = context.getElementById(selector.substring(1));
	} else if(classSelectorRE.test(selector)) {
		element = context.getElementsByClassName(selector.substring(1))[0];
	} else if(tagSelectorRE.test(selector)) {
		element = context.getElementsByTagName(selector)[0];
	}
	return element;
};

这里关键的地方，在于处理多级选择器。不过，这里没有处理同级多类型选择器的情况，后续会补充。