Allow more intuitive config of multiple domains

Currently, the Zend Ldap component can connect to multiple domains, as specified in the [docs][1]. However, it is left to the wrapping code to loop through the list of domains. I propose allowing the `Ldap` class to be constructed with an array of domains instead. Each of these domains would have the same options that `Ldap` uses now, but `Ldap` would do the looping internally, providing built-in failover and allowing more intuitive config files. I made sure not to break BC for the many cases when there is just one domain.

For example, see the difference in the wrapper before and after the addition.

Before (example from the docs):

```php
$multiOptions = array(
    'server1' => array(
        'host'                   => 's0.foo.net',
        ...
    ),
    'server2' => array(
        'host'                   => 'dc1.w.net',
        ...
    ),
);

$ldap = new Zend\Ldap\Ldap();

foreach ($multiOptions as $name => $options) {

    echo "Trying to bind using server options for '$name'\n";

    $ldap->setOptions($options);
    try {
        $ldap->bind($acctname, $password);
        $acctname = $ldap->getCanonicalAccountName($acctname);
        echo "SUCCESS: authenticated $acctname\n";
        return;
    } catch (Zend\Ldap\Exception\LdapException $zle) {
        echo '  ' . $zle->getMessage() . "\n";
        if ($zle->getCode() === Zend\Ldap\Exception\LdapException::LDAP_X_DOMAIN_MISMATCH) {
            continue;
        }
    }
}
```

After:
```php
$ldap = new Zend\Ldap\Ldap($multiOptions);
```

  [1]: http://framework.zend.com/manual/2.0/en/modules/zend.ldap.introduction.html