xtemplate: support async command

[yiminghe]

it('can combine inline command and block async command', function () {
    var tpl = '{{#async(1)}}{{upperCase(asyncContent)}}{{/async}}';
    var ret = '';
    expect(new XTemplate(tpl, {
        commands: {
            async: function (scope, option, buffer) {
                var newScope = new this.Scope();
                newScope.setParent(scope);
                return buffer.async(function (asyncBuffer) {
                    setTimeout(function () {
                        newScope.setData({
                            asyncContent: option.params[0] + ' ok'
                        });
                        option.fn(newScope, asyncBuffer).end();
                    }, 100);
                });
            },
            'upperCase': function (scope, option) {
                return option.params[0].toUpperCase();
            }
        }
    }).render({ }, function (error, content) {
            ret = content;
        })).toBe('');
    waitsFor(function () {
        return ret;
    });
    runs(function () {
        expect(ret).toBe('1 OK');
    });
});