Update lib/WWW/YouTube/Download.pm

Added that the YouTube video user is parsed out and offered as yet another substitution variable in file naming.
Create video_user.t

Added tests for parsing out of YT user(names), untested, meant as a starting point.
Update t/private/video_user.t

Was a mock up, now works.
Also: we should ask users if it is okay to connect to the Internet for testing!
Merge pull request #7 from isync/master

New substitution variable {user}