Update session processing

### 1. GET /api/session 削除

### 2. POST /api/session にuserを含む
sending
```json
{
  "email": "guest@ec.hokudai.ac.jp",
  "password": "guest"
}
```
returns
```json
{
    "logged_in": true,
    "token": "c70feb5d492c77a6daaaac034c56254f7528f14f9fe89fab1d3582df69182fcf",
    "user": {
        "id": 2,
        "email": "guest@ec.hokudai.ac.jp",
        "full_name": "guest guest",
        "handle_name": "guest",
        "birthday": "1990-01-01",
        "email_mobile": "guest@example.com",
        "admin": false,
        "graduation_year_id": 1,
        "errors": {}
    }
}
```

### 3. logoutは常に200を返す
with valid token
```json
{
    "logged_out": true
}
```
withOUT valid token
```json
{
    "logged_out": false
}
```

### 4. `rake db:dev`で非adminユーザー生成
email: guest@ec.hokudai.ac.jp
password: guest