Fix #38: Properly initialize NSS, RXTX, RST pins

[manuelbl]

Here's a fix for issue #38 (Need to initialize nss pin to HIGH).

• writeDigital is deliberately called before pinMode(..., OUTPUT) to avoid spikes on the output pin. This pattern should work on all Arduino implementations.
• RXTX is initialized to RX mode.
• RST is set to floating. That's the same state the pin is in after the reset has been executed. Before it was implicit set to LOW.

[terrillmoore]

Looks good, thanks. However, users must be aware that (1) this may increase power consumption, as there may be module-specific pull-ups or pull-downs on some signals; (2) this doesn't solve the problem of setting NSS high in those situations where the LMIC hasn't been initialized. I'm not sure whether #38 might have been reported in that context. We also need an update to README.md, which I'll add manually after the merge.