When trying to download some GameKings videos, not all worked. This was because not all videos had a "/large"-URL available. The extractor checks now if the /large URL is available, if it isn't, it tries to get the normal URL.
Player-url only needed for rmftp, not for regular URLs
Fixed code conventions (mainly adding two or more spaces before making an inline comment)
Implements #3638
720p @ 1000KB/s looks way better than 1080p @ 500KB/s
'' might actually be passed in, so check for None.
Conflicts: youtube_dl/downloader/external.py
Passing source address option to external downloaders