Create a BoltItem interface which is used to make
the database fucntions more generic.
As part of this change, the Status and Card types
have migrated back into the board package.
Add database functions to read and write statuses to the status bucket
in a BoltDB database.
Add database functions to read and write cards to the card bucket.
Use encoding/gob to encode the Status and Card values before writing
them to the database.
