CrossAuth: Authenticate users in multiple domains at once

This class can be used to authenticate users in multiple domains at once.

It generates HTML with Javascript tags for one domain that also submits the same user name and password to the authentication page of another domain when the user is successfully authenticated in the main domain.

The class does not perform the actual verification of the submitted user name and password. It just provides means to specify an external callback function that can be customized to perform that verification.