Function login

login(username, password, url?): Promise<null | KosServiceResponse<LoginResponse>>
Attempts a log in against the specified URL.
Parameters
- username: string
  the username of the user to log in
- password: string
  the password of the user to log in
- url: string = ...
  the url to send the login request to.
Returns Promise<null | KosServiceResponse<LoginResponse>>
a login response with the token of the logged in user
Default Value
```
- defaults to the Admin server URL.
```