Add an after_ldap_authentication callback

This adds a `after_ldap_authentication` callback similar to the
[after_database_authentication](http://rubydoc.info/github/plataformatec/devise/master/Devise/Models/DatabaseAuthenticatable#after_database_authentication-instance_method)
callback in `DatabaseAuthenticatable`.
Merge pull request #170 from blt04/add-after-callback

Add an after_ldap_authentication callback