Improve Credits.add function

Changed the Credits.add() function so that it's parameters follow the same conventions as the other functions in credits.js, meaning that data is now passed via a single data parameter, instead of amount, statement, etc, each having its own parameter.

This gets rid of the check to see if description was specified any more, allowing adding more parameters to be future-proof (no need to cause breaking changes by adding more parameters..)

By extension, other data can be fed to the function, such as "appears_on_statement_as"