[youtube] Add and extractor for the subscriptions feed (closes #498)

It can be downloaded using the ytsubscriptions keyword.
It needs the login information.
release 2013.07.07
[youtube:subscriptions] Use colon for differentiation of shortcuts
release 2013.07.07.01
YoutubeIE: extract the thumbnail in the best possible quality
VimeoIE: authentication support (closes #885) and add a method in the base InfoExtractor to get the login info
[3sat] Add support (Fixes #1001)
Merge branch 'master' of github.com:rg3/youtube-dl
[arte] Always look for the JSON URL (Fixes #1002)
release 2013.07.08
[archive.org] Add extractor (Fixes #1003)
release 2013.07.08.1
Do not redefine variables in list comprehensions
Remove dead code
YoutubeIE: use the new method in the base IE for getting the login info
YoutubeSubscriptionsIE: raise an error if there's no login information.
added an IE and test for dotsub.
Add VeohIE (closes #1006)
Merge pull request #1009 from yasoob/master

Added an IE and test for dotsub.com videos. ( closes #1008 )
DotsubIE: simplify and extract the upload date

Do not declare variables for fields in the info dictionary.