Fix `add` to handle integer values (casted to string) (IntegrityError: UNIQUE constraint failed)

Handle non-string (integer) input to `add`, by casting it to a string,
and build up `str_tags` and `tag_objs` in a single loop.

This fixes the following error (from the added test):

    Traceback (most recent call last):
      File "venv/src/django-taggit/tests/tests.py", line 94, in test_integers
        orange.tags.add(1)
      File "venv/src/django-taggit/taggit/utils.py", line 128, in inner
        return func(self, *args, **kwargs)
      File "venv/src/django-taggit/taggit/managers.py", line 145, in add
        tag_objs.add(self.through.tag_model().objects.create(name=new_tag))
      File "venv/lib/python2.7/site-packages/django/db/models/manager.py", line 157, in create
        return self.get_queryset().create(**kwargs)
      File "venv/lib/python2.7/site-packages/django/db/models/query.py", line 322, in create
        obj.save(force_insert=True, using=self.db)
      File "venv/src/django-taggit/taggit/models.py", line 71, in save
        return super(TagBase, self).save(*args, **kwargs)
      File "venv/lib/python2.7/site-packages/django/db/models/base.py", line 545, in save
        force_update=force_update, update_fields=update_fields)
      File "venv/lib/python2.7/site-packages/django/db/models/base.py", line 573, in save_base
        updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
      File "venv/lib/python2.7/site-packages/django/db/models/base.py", line 654, in _save_table
        result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
      File "venv/lib/python2.7/site-packages/django/db/models/base.py", line 687, in _do_insert
        using=using, raw=raw)
      File "venv/lib/python2.7/site-packages/django/db/models/manager.py", line 232, in _insert
        return insert_query(self.model, objs, fields, **kwargs)
      File "venv/lib/python2.7/site-packages/django/db/models/query.py", line 1514, in insert_query
        return query.get_compiler(using=using).execute_sql(return_id)
      File "venv/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 903, in execute_sql
        cursor.execute(sql, params)
      File "venv/lib/python2.7/site-packages/django/db/backends/util.py", line 53, in execute
        return self.cursor.execute(sql, params)
      File "venv/lib/python2.7/site-packages/django/db/utils.py", line 99, in __exit__
        six.reraise(dj_exc_type, dj_exc_value, traceback)
      File "venv/lib/python2.7/site-packages/django/db/backends/util.py", line 53, in execute
        return self.cursor.execute(sql, params)
      File "venv/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 451, in execute
        return Database.Cursor.execute(self, query, params)
    IntegrityError: UNIQUE constraint failed: tests_officialtag.name