Permit users to be loaded by email, in addition to their uid

Sometimes, it's nice to be able to load users by their email address, rather than just their uid.

Example:

```php
     	$ldapUserProvider = $this->get ('imag_ldap.security.user.provider');
        $user = $ldapUserProvider->loadUserByEmail ('kate@codebykate.com');
```

I've committed a patch that permits this usage.