Remove 3D code (did anyone really use this? I don't think so).
Don't limit length of potential video ID.

If this really stripped it, it's highly unlikely this will fix the underlying
problem (ie. this isn't a video ID).
Reinstate comment in itags section
Return early in strip_to_video_id
Return None instead of an Exception when encountering unknown quality groups
Simplify getting available itags
Simplify group/itag detection
Remove old 3D warning code.
Use docopts instead of argparse
Remove unneeded installation of argparse on Travis CI.
Merge branch 'cleanup'