Fixes to Error Messages

Some error messages were fixed:

Fixed two incorrect references:

• HTTP 4XX errors are usually the fault of the user (generally bad KEY/TOKEN pairs), and not Trello's fault.

• For creating a card, the argument 3 is a list (and not a card).

• Also added some comments to the auth.lua module containing instructions on how to fetch the API key and token.

Changes introduced by 1.3.5:

• Card:SetDesc(txt) was brought back, so that you don't need to use Card:SetProperty("desc", txt)