Add support for optionally following referrals

In some Active Directory setups, referrals are very common, which makes using this library pretty difficult. To solve this, we can tell PHP to automatically follow any referrals that are returned by a server, per the documentation at http://www.php.net/manual/en/ldap.constants.php.
Switch $followReferrals to an int, since PHP is picky.
Merge pull request #19 from cweagans/master

Support automatically following referrals