Adds support for passing a referer.
Updated README
Merge pull request #801 from expleo/add_referer_support
YoutubePlaylistIE: don't crash with empty lists (related #808)

The playlist_title wasn't initialized.
SteamIE: accept urls with agecheck
Allow to select videos to download by their upload dates (related #137)

Only absolute dates.
Fix some IEs that didn't return the uploade_date in the YYYYMMDD format

Create a function unified_strdate in utils.py to fix these problems
 Split FileDownloader in two classes: FileDownloader and YoutubeDL

YoutubeDL is the class that coordinates everything
FileDownloader gets a filename and an info dict and downloads the video.