Resolves #340 Connection refactoring

lib/auth_manager.class.dart

@@ -0,0 +1,44 @@
+part of 'main.dart';
+
+class AuthManager {
+
+  static final AuthManager _instance = AuthManager._internal();
+
+  factory AuthManager() {
+    return _instance;
+  }
+
+  AuthManager._internal();
+
+  Future getTempToken({String httpWebHost, String oauthUrl}) {
+    Completer completer = Completer();
+    final flutterWebviewPlugin = new FlutterWebviewPlugin();
+    flutterWebviewPlugin.onUrlChanged.listen((String url) {
+      if (url.startsWith("http://ha-client.homemade.systems/service/auth_callback.html")) {
+        String authCode = url.split("=")[1];
+        Logger.d("We have auth code. Getting temporary access token...");
+        Connection().sendHTTPPost(
+            host: httpWebHost,
+            endPoint: "/auth/token",
+            contentType: "application/x-www-form-urlencoded",
+            includeAuthHeader: false,
+            data: "grant_type=authorization_code&code=$authCode&client_id=${Uri.encodeComponent('http://ha-client.homemade.systems/')}"
+        ).then((response) {
+          Logger.d("Gottemp token");
+          String tempToken = json.decode(response)['access_token'];
+          Logger.d("Closing webview...");
+          flutterWebviewPlugin.close();
+          completer.complete(tempToken);
+        }).catchError((e) {
+          flutterWebviewPlugin.close();
+          completer.completeError({"errorCode": 61, "errorMessage": "Error getting temp token"});
+          Logger.e("Error getting temp token: ${e.toString()}");
+        });
+      }
+    });
+    Logger.d("Launching OAuth...");
+    eventBus.fire(StartAuthEvent(oauthUrl));
+    return completer.future;
+  }
+
+}