Add Not Rated content rating

Author: pkkid

tests/conftest.py

@@ -36,7 +36,7 @@
 AUDIOLAYOUTS = {'5.1', '5.1(side)', 'stereo'}
 CODECS = {'aac', 'ac3', 'dca', 'h264', 'mp3', 'mpeg4'}
 CONTAINERS = {'avi', 'mp4', 'mkv'}
-CONTENTRATINGS = {'TV-14', 'TV-MA', 'G', 'NR'}
+CONTENTRATINGS = {'TV-14', 'TV-MA', 'G', 'NR', 'Not Rated'}
 FRAMERATES = {'24p', 'PAL', 'NTSC'}
 PROFILES = {'advanced simple', 'main', 'constrained baseline'}
 RESOLUTIONS = {'sd', '480', '576', '720', '1080'}