Add infrastructure for paged lists

This commit allows to download pages in playlists as needed instead of all at once.
Before this commit,
    youtube-dl http://www.youtube.com/user/ANNnewsCH/videos --playlist-end 2 --skip-download
took quite some time - now it's almost instantaneous.
As an example, the youtube:user extractor has been converted.
Fixes #2175
Merge branch 'paged-lists'

Conflicts:
	test/test_utils.py
	youtube_dl/extractor/youtube.py