Initial LDAP support

Crude LDAP support. First you run the nt_user_ldap_sync.pl script to add the users from your ldap to the nictool db. This script will also cleanup any users that can't be found on ldap anymore. Then you can login.
Adding the code in here makes it so that Net::LDAP is now a requirement in order to install and use NicTool. That's not good.

Instead, put the LDAP password checking into it's own subroutine, and put a call to that subroutine in here. Add a preference to the nt_options table, and if that option isn't set, don't try LDAP auth. Instead of 'use Net::LDAP', perform a test (eval "require Net::LDAP") to see if the module is available, and if not, log an error and fall back to the build-in auth.