Per #14 add first/last fragment, debugging cleanup

Per #14 and
https://github.com/TMRh20/RF24Network/commit/e34a39472ba06ecf867aace81be4098d29c878d8#commitcomment-7727886
added NETWORK_FIRST_FRAGMENT
- Add total_fragments to RF24NetworkFrame
- Add error checking per discussion
- Modify debug to include FRG for fragment handling messages and RT for
routed messages
- Add SERIAL_DEBUG_MINIMAL to be changed as required for debugging new
features in RF24NetworkDEV