- Replace CFE_SB_RcvMsg with CFE_SB_ReceiveBuffer
- Use CFE_SB_Buffer_t for receiving and casting to command types
- Use CFE_MSG_CommandHeader_t and CFE_MSG_TelemetryHeader_t in
  command and telemetry type definitions
- Use CFE_SB_TransmitMsg to copy the command and telemetry
  into a CFE_SB_Buffer_t and send it where needed
- Avoids need to create send buffers within the app (or union
  the packet types with CFE_SB_Buffer_t)
- Eliminates references to CFE_SB_CmdHdr_t and CFE_SB_TlmHdr_t
  that formerly enforced alignment since these had potential
  to change the actual packet sizes
- No need to cast to CFE_MSG_Message_t anywhere since it's
  available in the CFE_SB_Buffer_t union
- Replaced CFE_MSG_Size_t with size_t

Fix #66, Implement message alignment pattern