Put Pactas data in configuration file.

@hajoeichler I make a pull request because once merged the documentation will be inconsistent with the code.

So in order to fix it, this line of code should be:
"To change to your Pactas account, modify (client ID -> pactas.clientId) and (client secret -> pactas.clientSecret) (into -> ??) in (app/utils/pactas/Pactas.java -> conf/application.conf)"

The Paymill explanation is also wrong, as the Paymill key is located in the Pactas backend, not in our code:
"To change to your PAYMILL account, enter your public key in (app/views/main.scala.html (line ~52) -> Pactas backend!!)"

Also note the Pactas credentials are exposed.