Implement Password Modify Extended Operation

[Zeault]

I implemented the LDAP Password Modify Extended Operation. Completes #127 and #143.

[Zeault]

I did not add any new configuration options for this feature. Instead, the change password code will now check to see if the extended operation is supported by the server and use it automatically.

This works seamlessly on OpenLDAP because my tests have shown that a user that has permission to write a password will necessarily have permission to change it with the extended operation. However, on other LDAP servers it is possible that this breaks someone's working configuration. It all depends on how the server handles permissions.