Use the 'extractor_key' field for the download archive file

It has the same value as the ie_key.

Reported in #1798

Allow to initialize a YoutubeDL object without parameters

Fixes #1828

Having to pass the 'outtmpl' parameter feels really strange when you just want to extract the info of a video.

It doesn't respect the 'Range' header.

Allow download of http://goo.gl/7X5tOk
Fixes #1818

This also comes with a lot of extra format fields
Fixes #1518

It's a subclass of MTVIE

The extractor for colbertnation.com and thedailyshow.com is called now ComedyCentralShowsIE

Also provide a better error message for geoblocked videos.

Uses a new helper method in InfoExtractor: _download_xml