Add activeUrl Public Method to MigrateSourceXML in Migrate

Motivation

For my own projects, I have had to had to add a public method to the MigrateSourceXML class to provide the current activeUrl. I have an install profile that includes the migrate project and it breaks my custom migrate module.

Proposed resolution

I have attached a patch that adds the requested functionality. I can't guarantee that this is the best way to accomplish what I am trying to do, so I'll leave it to the maintainers to decide whether or not this functionality is worthwhile or not. Either way, I'll be able to apply this patch in my drush make file now.

Thanks for such an awesome module!