tags: properly dealloc tags

There were two memory leaks: we were not closing the git_tag object,
and we were not decreasing the refcount on the repo.

This finishes fixing issue #20

pygit2.c

@@ -1658,7 +1658,9 @@ static PyTypeObject BlobType = {
 static void
 Tag_dealloc(Tag *self)
 {
+    git_tag_close(self->tag);
     Py_XDECREF(self->target);
+    Py_XDECREF(self->repo);
     Py_TYPE(self)->tp_free((PyObject*)self);
 }
 

test/test_tag.py

@@ -54,7 +54,6 @@ class TagTest(utils.BareRepoTestCase):
         self.assertEqual('Tagged root commit.\n', tag.message)
 
         commit = tag.target
-        del tag
         self.assertEqual('Initial test data commit.\n', commit.message)
 
     def test_new_tag(self):