login by POST request and logIn-> login (#347)

* login by POST request

* Add login demo

Author: wangxiao

demo/test-es6.js

@@ -59,3 +59,8 @@ query.equalTo('name', 'hjiang');
 query.find().then((list) => {
   console.log(list);
 });
+
+// 用户登录
+AV.User.login('ttt', '123456')
+.then((res) => console.log(res))
+.catch(err => console.log(err));

src/user.js

@@ -358,7 +358,7 @@ module.exports = function(AV) {
      */
     logIn: function(options) {
       var model = this;
-      var request = AVRequest("login", null, null, "GET", this.toJSON());
+      var request = AVRequest('login', null, null, 'POST', this.toJSON());
       return request.then(function(resp, status, xhr) {
         var serverAttrs = model.parse(resp, status, xhr);
         model._finishFetch(serverAttrs);