Added identity field funcionality and its unit test.

Hi! I have seen that there is a param in yaml config to set "user_identity_field" but this method forces the field to be "username".
Personally I need a token with "email" and not "username", I have rewritten this method using ReflectionClass...

With this code, the "user_identity_field" is full configurable.
Thanks!

Hi :) maybe a PropertyAccess would be more convenient than a reflection, it could allow to pass a path in `$this->userIdentityField`.

```php
$accessor = \Symfony\Component\PropertyAccess\PropertyAccess::createPropertyAccessor();
$payload[$this->userIdentityField] = $accessor->getValue($user, $this->userIdentityField);
```