Allow playing videos using the video ID
Now the "play_video" action can be called either:
- with a "url" argument which value is the complete URL of a video (containing the instance, the video ID and the resolution)
- or with "instance" and "id" as parameter. In this case the complete URL containing the resolution will be retrieved. It will allow other add-ons to play videos since the available resolutions are not known.
Other changes:
- improve the management of errors when retrieving the information of a video
- fix some other PEP 8 errors