[youtube] Download DASH manifest

If given, download and parse the DASH manifest file, in order to get ultra-HQ formats.
Fixes #2166
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
[southparkstudios] Use the generic `_real_extract` provided by the base class
Merge pull request #2195 from dstftw/master

[space] Add support for mobile URLs
[comedycentral] Use the generic `_real_extract` provided by the base class
Add new --default-search option (#2193)
release 2014.01.22.2
Add -f bestaudio (Fixes #2163)
Use unicode_literals in test_YoutubeDL
Default to -f best-audio when only audio is requested
release 2014.01.22.3
Document -f worstaudio as well
[cspan] Disable test

It works fine from all my machines, no matter where, but from travis, we get lots of 403s.
Maybe another project is scraping CSPAN from travis and they're blocking the travis machines?
[xvideos] Fix thumbnail extraction

Signed-off-by: Philipp Hagemeister <phihag@phihag.de>
[xvideos] Simplify
Merge branch 'youtube-dash-manifest'

Conflicts:
	youtube_dl/extractor/youtube.py
Merge branch 'paged-lists'

Conflicts:
	test/test_utils.py
	youtube_dl/extractor/youtube.py
release 2014.01.22.4
[newgrounds] Simplify
Add __len__ to PagedLists