nRF24L01 support

I have a use case for some small battery-powered distributed sensors. Ideally this would be a good use for BLE or [Z-Wave](http://www.z-wave.com/shop-z-wave-smart-home-products/category/smart-sensors), but BLE support has a ton of overhead and Z-Wave is [proprietary and expensive](https://www.amazon.com/dp/B01IRBDBZY).

What I'm probably going to end up doing is setting up a few devices with STM32s that will spend most of their time in suspend, occasionally waking to make a sensor reading and send it off via a [nRF24L01](https://platformio.org/lib/show/433/RF24) radio if conditions are met. On the ESP side, it would just be a matter of wiring up the RF24 SPI/Enable/IRQ pins to GPIOs.

The chip is limited in hardware to 32 byte message payloads. Perhaps something like this:
```c
struct payload {
unit8_t type;
char unit[3];
char value[14];
char name[14];
};
```

Ideally, esphomelib would not need to define a list of sensors ahead of time, but simply reformat and pass the messages through to MQTT.