Add bh1750 usage example

examples/bh1750/Makefile

@@ -0,0 +1,3 @@
+PROGRAM=bh1750_example
+EXTRA_COMPONENTS = extras/i2c extras/bh1750
+include ../../common.mk

examples/bh1750/bh1750_example.c

@@ -0,0 +1,41 @@
+#include <stdio.h>
+
+#include "espressif/esp_common.h"
+#include "esp/uart.h"
+
+#include "FreeRTOS.h"
+#include "task.h"
+
+#include "i2c/i2c.h"
+#include "bh1750/bh1750.h"
+
+#define SCL_PIN 5
+#define SDA_PIN 4
+
+static void measure(void *pvParameters)
+{
+    bh1750_configure(BH1750_ADDR_LO,
+                     BH1750_CONTINUOUS_MODE | BH1750_HIGH_RES_MODE);
+
+    while (1) {
+        while(1) {
+            vTaskDelay(200 / portTICK_PERIOD_MS);
+            
+            printf("Lux: %d\n", bh1750_read(BH1750_ADDR_LO));
+        }
+    }
+}
+
+void user_init(void)
+{
+    uart_set_baud(0, 115200);
+
+    // Just some information
+    printf("\n");
+    printf("SDK version : %s\n", sdk_system_get_sdk_version());
+    printf("GIT version : %s\n", GITSHORTREV);
+
+    i2c_init(SCL_PIN, SDA_PIN);
+
+    xTaskCreate(measure, "measure_task", 256, NULL, 2, NULL);
+}