fix proper format selection given only extension

if video formats are not seeded through available formats, resulting url is sometimes nil even while valid format existed

  For instance: ViddlRb::Youtube.get_urls_and_filenames("http://www.youtube.com/watch?v=WawU4ouldxU",{:quality => {:extension => "mp4"}})
  (video has valid format 22, but returns nil because it finds non-existent 38 first)
update youtube formats list

according to http://stackoverflow.com/questions/18876872/how-to-get-the-lowest-quality-youtube-video
add some formats from wikipedia
returned default format order array for 1.8.7
Merge pull request #81 from snusmu/master

youtube plugin update