Update youtube-dl to 20100804.
Requested by PR#43761 from Kamel Derouiche.

* Update user-agent string
* Fix metacafe.com code not working due to gdaKey again (fixes issue #185)
* Merge Gavin van Lelyveld's patch for --playlist-start option
* Put back -b option as a placeholder with a warning message
* Consider the file downloaded if the size differs in less than 100 bytes
  (fixes issue #175)
* Reorganize request code to make it a bit more robust
* Properly detect YouTube error messages to print them on screen (fixes issue
  #172)

(obache)