Add a way to specify DO NOTHING.

If `None` is specified as patterns, that mean do nothing.
e.g.

    thumbnail = ThumbnailField('thumbnail', upload_to='thumbnails', patterns = {
            'original': None,
        }
Bump version
Merge branch 'master' of github.com:lambdalisue/django-thumbnailfield
Merge branch 'master' of github.com:lambdalisue/django-thumbnailfield