Release note review

Review edits for https://github.com/python-pillow/Pillow/pull/1032

 * English changes.
 * Renamed filename to 2.7.0 because we may have a 2.7.1.
 * Similarly, refer to 2.7.0 in the document rather than 2.7, and "Pillow 2.7.0" just so there's no confusion with Python 2.7.x.