A suitable downloader can be found using the 'get_suitable_downloader' function.
Each subclass implements 'real_download', for downloading an info dict you call the 'download' method, which first checks if the video has already been downloaded
A string statement is not a docstring if it doesn't occur right at the top
of modules, functions, class definitions etc.
This patch fixes it.
Signed-off-by: Rogério Brito <rbrito@ime.usp.br>