esp8266: Add basic I2C driver, with init and writeto methods.

Tested and working with SSD1306 I2C display.
diff --git a/esp8266/modpyb.h b/esp8266/modpyb.h
index 095bc01..e691805 100644
--- a/esp8266/modpyb.h
+++ b/esp8266/modpyb.h
@@ -1,6 +1,7 @@
 extern const mp_obj_type_t pyb_pin_type;
 extern const mp_obj_type_t pyb_adc_type;
 extern const mp_obj_type_t pyb_rtc_type;
+extern const mp_obj_type_t pyb_i2c_type;
 
 typedef struct _pyb_pin_obj_t {
     mp_obj_base_t base;