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。