[metacafe] Fix support for age-restricted videos (fixes #1696)

The 'Content-Type' header must be set for disabling the family filter.
The 'flashversion' cookie  is only needed for AnyClip videos.
Added tests for standard metacafe videos and for age-restricted videos.
Also set the 'age_limit' field.
Add an extractor for vk.com (closes #1635)
[youtube] Encode subtitle track name in request (Fixes #1700)
release 2013.11.02