User registration

### Process

#### 1. push JSON `http://hokui.net/api/users/create`
```json
{
  "user": {
    "email": "foobar@example.com",
    "password": "hogefuga",
    "family_name": "foo",
    "given_name": "bar",
    "handle_name": "foobar",
    "birthday": "1990-01-01",
    "email_mobile": "hogefuga@example.com",
    "class_year": 93
  }
}
```
`email_mobile` is optional.
If success, receive `HTTP 201` and proceed below.
If erroneous, receive `HTTP 422`

#### 2. receive email with activation URL
`http://hokui.net/users/activate/:email_local/:activation_token`

#### 3. access URL
If success, redirect to `http://hokui.net` with query parameter `?activation_success=true`
If erroneous, query parameter will be `?activation_success=false`