Implement isScanning api (#188)

* Implement isScanning api

* Improve IOS scanning api

* Update Readme and Changelog

test/universal_ble_test_mock.dart

@@ -95,4 +95,9 @@ abstract class UniversalBlePlatformMock extends UniversalBlePlatform {
       BleOutputProperty bleOutputProperty) {
     throw UnimplementedError();
   }
+
+  @override
+  Future<bool> isScanning() {
+    throw UnimplementedError();
+  }
 }