Add initial sphinx docs

With an initial guide for using youtube_dl from python programs.
Add an option to specify custom HTTP headers
toypics.net support
itag 160 is 144p, not 192p
Make missing test definition fields an error

If the result is not testable (for example, because a description changes often), either pass in a type or a regular expression (a string starting with 're:')
Merge remote-tracking branch 'hurda/patch-1'
[youtube] Remove useless resolution fields from format definitions

These can be - and are - calculated automatically by the YoutubeDL core.
Merge remote-tracking branch 'fiocfun/toypics-support'
[toypics] Separate user and video extraction (#2601)
Remove unused imports and clarify variable names
[pornhub] Fix uploader extraction and extract counts
[xtube] user playlist extractor
[metacafe] Modernize
[metacafe] Replace cbs test
[metacafe] More modernize
[metacafe] Remove accidently inserted comment string
Merge remote-tracking branch 'fiocfun/xtube-user-extractor'
[worldstarhiphop] Modernize
[xbef] (Add extractor)
[utils] Simplify setproctitle