[soundcloud] Replace 404 test
[tutv] Replace 404 test and modernize
[youtube & jsinterp] Fix signature extraction (fixes #3255)

Some functions are defined now inside an object, the jsinterp will search its definition if the variable is not defined in the local namespace.
release 2014.07.15
Remove binary and auto-updating entry point
Remove update functionality
Update UA
Wait ~1 sec between HTTP requests to avoid getting CAPTCHAed by Google
For YouTube videos we already have, skip without even requesting video info
When continuing a YouTube download with --continue, skip it if the directory has any download with the same ID, even if it has a different filename.
Get directory listing only once
Really don't listdir more than once
Fix startup
Remove _ffmpeg_filename_argument obsoleted by new ffmpeg; delete DASH input files after merging; fix merging on Windows by using short paths; use ffmpeg output filename that doesn't look like a finished video until ffmpeg succeeds
Fix mtime after merging
Fix FFmpegPostProcessor for non-Windows
Add self._ffmpeg_filename_argument back to not break with avconv