add simple http invoker

src/main/java/com/virjar/ratel/api/SimpleHttpInvoker.java

+package com.virjar.ratel.api;
+
+import android.util.Log;
+
+import java.io.InputStream;
+import java.net.HttpURLConnection;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+
+import external.org.apache.commons.io.IOUtils;
+
+public class SimpleHttpInvoker {
+    public static String get(String url) {
+        try {
+            HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
+            connection.setRequestMethod("GET");
+            connection.connect();
+            int responseCode = connection.getResponseCode();
+            if (responseCode != HttpURLConnection.HTTP_OK) {
+                connection.disconnect();
+                return null;
+            }
+            try (InputStream inputStream = connection.getInputStream()) {
+                return IOUtils.toString(inputStream, StandardCharsets.UTF_8);
+            } finally {
+                connection.disconnect();
+            }
+        } catch (Exception e) {
+            Log.e(RatelToolKit.TAG, "error for url:" + url, e);
+            return null;
+        }
+    }
+
+
+}