Support for public boards

I didn't see support for accessing public boards with this api. (e.g.,https://trello.com/b/nC8QJJoZ/trello-development).

Added a few small changes to allow that. If you don't pass in a token, it now assumes you are trying to open a public board, and will attempt to do so. (Although it may be best to issue some sort of warning, or make them pass in a "public_boards = True" to TrelloClient or something?)

These changes just let you view actions with a public board.