Re-add coconut specific handling

Updates devise_ldap_authenticatable to the latest github version and re-add specific handling for AD and some convenience method which is used in Coconut.

Added methods:
Devise::LDAP::Adapter#valid_user?
Devise::LDAP::Connection#valid_user?
Devise::LDAP::Connection#encode_for_ad(password)

Added config option:
Devise.ldap_is_ad

Removed method:
Devise::LDAP::Adapter#update_own_password

Changed method:
Devise::LDAP::Connection#change_password!
Devise::LDAP::Adapter#update_password