Added an API method for retrieving the padID from the readonlyID

Added an API method for retrieving the padID from the readonlyID

Shold fix Issue https://github.com/ether/etherpad-lite/issues/321.
I choose to implement the changes done by disky-mk again because i was unable to find out how to pull his branch into the main etherpad tree. I mentioned that it is mainly his work tho.