save-images command is broken in Python 2.7 #174

Bug/Issue Description:
PySceneDetect crashes when using the save-images command under Python 2.7.

Expected Behavior:
save-images should work properly :)

Computing Environment:
N/A, affects all platforms running Python 2.7.

Sample Output
scenedetect -i '/home/santiago/Documents/NewConsultoriaMarketing/filename.mkv' detect-content save-images

[PySceneDetect] Loaded 1 video, framerate: 29.97 FPS, resolution: 1280 x 720
[PySceneDetect] Downscale factor set to 5, effective resolution: 256 x 144
[PySceneDetect] Image output format set: JPEG
[PySceneDetect] Detecting scenes...
100%|████████████████████████████████| 9680/9680 [00:24<00:00, 388.39frames/s]
[PySceneDetect] Processed 9680 frames in 24.9 seconds (average 388.35 FPS).
[PySceneDetect] Detected 9 scenes, average shot length 35.9 seconds.
[PySceneDetect] Comma-separated timecode list:
00:00:00.734,00:00:01.802,00:03:33.146,00:03:38.652,00:04:21.094,00:05:18.752,00:05:19.886,00:05:20.954
[PySceneDetect] Downscale factor set to 1, effective resolution: 1280 x 720
[PySceneDetect] Generating output images (3 per scene)...
0%| | 0/27 [00:00<?, ?images/s]Traceback (most recent call last):
File "/home/santiago/.virtualenvs/videotracking/bin/scenedetect", line 8, in
sys.exit(main())
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/scenedetect/main.py", line 58, in main
cli.main(obj=cli_ctx) # Parse CLI arguments with registered callbacks.
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/click/core.py", line 792, in main
ctx.exit()
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/click/core.py", line 404, in exit
self.close()
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/click/core.py", line 494, in close
cb()
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/scenedetect/cli/context.py", line 397, in process_input
output_dir=self.image_directory)
File "/home/santiago/.virtualenvs/videotracking/local/lib/python2.7/site-packages/scenedetect/cli/context.py", line 225, in _generate_images
for start, end in scene_list
AttributeError: 'list' object has no attribute 'stop'
0%| | 0/27 [00:00<?, ?images/s]

Thank you @santiagodemierre for reporting this!