ESP8266 to ESP8266 i2c

With added support for i2c slave from [here](https://github.com/esp8266/Arduino/compare/master...bjoham:i2c_slave) by @bjoham, after modifying all files.

Master (ESP8266)
```
#include

int x = 3;
void setup() {

Wire.pins(4,5);
Wire.begin(); // join i2c bus (address optional for master)
Wire.beginTransmission(8); // transmit to device #8
Wire.write(x); // sends one byte
Wire.endTransmission(); // stop transmitting
}

void loop() {

}
```

Slave (ESP8266)
```
#include

int x = 3;

void setup() {
pinMode(D0,OUTPUT);
Wire.pins(4,5);
Wire.begin(8); // join i2c bus with address #8
Wire.onReceive(receiveEvent); // register event
//Serial.begin(9600); // start serial for output
// Serial.println("Got something?");
digitalWrite(D0,LOW);
}

void loop() {
delay(100);
if (x == 3) {
digitalWrite(D0, HIGH);
}
//If value received is 3 blink LED for 400 ms
if (x == 0) {
digitalWrite(D0, LOW);
}
}

// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany) {
x = Wire.read(); // read one character from the I2C'
}
```

I have connected pins D1,D2,GND on both the boards(NodeMCU).

This doesn't seem to work. Any help will be appreciated.