feat(reference): add base bindings and api

example/reference_example.dart

@@ -0,0 +1,14 @@
+import 'dart:io';
+
+import 'package:libgit2dart/libgit2dart.dart';
+
+void main() {
+  final repo = Repository.open(Directory.current.path);
+  final ref = Reference.lookup(repo, 'refs/heads/master');
+
+  print('Reference SHA hex: ${ref.target}');
+  print('Is reference a local branch: ${ref.isBranch}');
+  print('Reference full name: ${ref.name}');
+
+  ref.free();
+}