LdapSource read() fixes: conditions, offsets, and counts

[ctgraham]

Fix LdapSource read queries to properly handle conversion of conditions array to LDAP filter, to support offsets, and to properly handle counts (though still a hack).

• Replace broken _conditionsArrayToString() and mostly broken _conditions() with a single working _conditionsArrayToString()
• If an offset was passed to a search, add this into the limit, then filter out the desired records after retrieval; otherwise, Cake's paginator doesn't work.
• Use the 'COUNT(*) AS ' . $this->name('count') hack consistently.