extractor: youtube: Set extension of AAC audio formats to m4a.

This, in particular, eases downloading both audio and videos in DASH formats
before muxing them, which alleviates the problem that I exposed on issue

Furthermore, one may argue that this is, indeed, the case for correctness's
sake.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
extractor: youtube: Fix extension of dash formats.

While we are at it, separate the audio formats from the video formats.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
extractor: Set age limit on some adult-related extractors.

This is similar in spirit to what was done in commit 8e590a117f.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Add an author email
[CinemassacreIE] Fix information extraction
extractor: Set age limit on some adult-related extractors.

More age limit of videos for adult-related sites.

Note that, for redtube, I explicitly left the variable containing the age
limit, since the comment justifying the age limit is a good thing to have.

That being said, I included the age limit field on the test, to better
reflect what the information extractor does (even if it may not break the
automated tests).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
[xhamster] Add support for age_limit (Instead of #1627)
Merge remote-tracking branch 'rbrito/set-age'

Conflicts:
	youtube_dl/extractor/xhamster.py
Accept requested formats to be in the format 35/best (closes #1552)

The format selection code is now an independent function.
Allow to use the extension for the format selection

The best format with the extension is downloaded.
YoutubeDL: remove method that came from FileDownloader
The 'format' field now defaults to '{format_id} - {width}x{height}{format_note}'

Following the YoutubeIE format. The 'format_note' gives additional info about the format, for example '3D' or 'DASH video'.
[arte] Set the format_note and the format_id fields (closes #1628)
[googleplus] Fix upload_date extraction
Fix the duration field for the VideoDetective and InternetVideoArchive tests

Also remove the use of the old format system and the comment
[youtube] Use 'node is None' when checking if the video has automatic captions

It had stopped working and it reports a FutureWarning
[vimeo] Implement the new format selection system (closes PR #996)

Rebased and deleted some parts to use the new system instead of copying the one from YoutubeIE
Respect the download parameter in YoutubeDL.process_video_result if the extractor handle the format selection
Style fixes in YoutubeDL.py

Fixed some of the problems reported by pep8
[nhl:videocenter] Fix playlist title extraction