Fix: Standby-I in sleep mode, other changes

Finalization of enhanced sleep mode:
* 1. The radio is configured to use Dynamic Payloads and ACK payloads
with Auto-Ack enabled
* 2. The radio enters PTX mode and attaches an interrupt handler to the
radio interrupt input pin (pin 2)
* 3. The radio uses the Watchdog Timer to awake at set 1 second
intervals in the example file
* 4. Every interval, it sends out a 'sleep' payload and goes back to
sleep. Incoming payloads will then be treated as ACK payloads, while the
radio remains in STANDBY-I mode.
* 5. If an interrupt is triggered, the radio wakes up
* 6. When a message is sent to the sleeping node, the interrupt triggers
a wake up, the MCU
* grabs the payload, and switches back to receive mode in case more data
is on its way.

Compared to receive mode:
* The radio will draw about 13.5 mA in normal receiver mode.
* In STANDBY-I mode, the radio draws .000022mA, and is able to awake
when payloads are received.

Changed Radio initialization:
- Uses new radio.maskIRQ function to disable TX and MAX_RT interrupts
for sleep mode

- Added code to discard sleep payloads
- Added function for custom initialization of watchdog timer intervals