Prevent unwanted card creation by authenticating requests.

While taking a look at what you'd built, I realised that it would be quite possible (and quite easy) for someone to create a card in the live Trello board associated with the running instance of this service. The fix is pretty simple, so I thought I'd just create a pull request, rather than just pointing it out.

A simple token is enough to ensure that people stumbling across this service cannot inadvertantly or maliciously create cards in the associated Trello board.