Added a "format" to messages

Messages can now be specified as being in standard (11-bit header) or extended format (29-bit header). If a PGN is specified, the message is set to extended, but if nothing is specified standard is assumed.

This is required for eventually adding message transmission.

Additionally this is required for proper filtering of messages based on format. Messages with the same ID that differ in format are actually considered different messages. This is the next thing to add now that format is being parsed.