Support for public boards

I didn't see support for accessing public boards with this api. (e.g., https://api.trello.com/1/boards/4d5ea62fd76aa1136000000c?key=158f77ec277ade783377b02ea80c3741).

Added some small changes so that tokens are now not required, and if it's not provided, it will attempt to open a public board with that ID.
Merge pull request #25 from naiyt/master

Support for public boards