Add basic --download-archive option

Often, users want to be able to download only videos they haven't seen before, despite the video files having been deleted or moved in the mean time.
When --download-archive FILE is given, the extractor and ID of every download is recorded in the specified file. If it is already present, the video in question is skipped.
Allow users to specify an age limit (fixes #1545)

With these changes, users can now restrict what videos are downloaded by the intented audience, by specifying their age with --age-limit YEARS .
Add rudimentary support in youtube, pornotube, and youporn.
[generic] Clarify error messages
Call extracted property age_limit everywhere
Merge branch 'age_limit'
Remove superfluous parenthesis
Merge branch 'download-archive'

Conflicts:
	youtube_dl/YoutubeDL.py
	youtube_dl/__init__.py
[redtube] add age_limit
[vimeo] Recognize urls ending in a slash (fixes #1242)
Merge branch 'master' of https://github.com/rg3/youtube-dl into 1321-download-annotations

Conflicts:
	youtube_dl/extractor/youtube.py