improve session

1. Save body info when login
===
````
POST /users/login
{
  "username": "johnsmith",
  "password": "password",
  "role": "owner"
}
```
`role` will be save into `ctx.session`

2. Access from On Event Script
===
add session variable on scriptContent

```
OnGet /users
session.count = session.count || 0;
session.count++;
```

Then we can 
```
OnPost /todos
console.log(session.count)
```