merging backend changes for login

To setup the project:
1. pull the changes
2. setup mongodb in your test system
3. create database 'zimolodb'
4. create document 'user'. the schema is 
user:{
        username
	firstname
	lastname
	email
	psw
}

5. run the project using node app.js
6.notice the funcitonalities of registration, login, logout all work