Completed RXPacket and TXPacket, started Serial class

RXPacket and TXPacket now have getters and setters for everything, a size() method, and the ability to convert between byte buffers and structs. Serial opens a UART stream and starts and stops a thread correctly but doesn't do anything in the thread yet.

Signed-off-by: Tushar Pankaj <tushar.s.pankaj@gmail.com>