Enable users to be loaded by Email address

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.