Added names() method to _TaggableManager

The method returns a (generator) list of just the tag names, so you can do things like this:

```python
if "my_tag" in obj.tags.names():
    # etc
```

This is much easier than other methods.

I originally submitted this as https://github.com/alex/django-taggit/pull/100 but accidentally deleted my repo. This time I've also added tests and used `values_list` like suggested.