Add UserService::loadByLogin

This PR adds the `loadByLogin` method to the `UserService`. No new tests are added, since  tests for `UserService::loadUserByCredentials` already cover the behavior.

## Usage:

```php

$repository    = $this->getRepository();
$userService = $repository->getUserService();

$userService->loadUserByLogin( 'admin' ); 
    #=>  \eZ\Publish\API\Repository\Values\User\User

```