Finalize serial.h and Serial examples

[serisman]

• serial.h needs to be finalized, i.e. cleaned up and made more extensible
  • Possibly allow serial_setup to be passed a baud rate to use, or have a compiler option defining BAUD_RATE passed in from the Makefile.
• The Serial examples need README files.
  • Serial_HelloWorld
  • ReadButton_WriteSerial