Improve transfer speeds

1. Only write if network.available() returns 0
2. Rewrite if writing to TUN device fails
3. Lower delay from 2 to 1ms
4. Disable debug by default
5. Change channel to 97 from 90
Merge branch 'TMRh20_Features'
Change default to TAP interface for testing

- Change default to TAP for testing with RF24Ethernet
- Drop payloads that exceed rate of interface
Update to work with RF24Ethernet

Add EXTERNAL_DATA_TYPE to outgoing payloads
Add auto-routing for RF24Ethernet

RF24Tun now handles ARP requests, and broacasts as required to find the
IP of related devices, using RF24Network multicast
- IP payloads are delivered according to MAC (RF24Network/RF24) address
- This allows multiple RF24Ethernet nodes to be connected entirely using
TCP/IP
Merge pull request #3 from TMRh20/master

Small changes to improve transfer rates. Set TAP as default device. RF24Ethernet routing