gpg keys should be cross-signed

[ghost]

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Gpg keys used to sign the releases on http://rg3.github.io/youtube-dl/download.html should be cross-signed by other mentioned keys to build a minimal trust chain for users. Especially for the newer key 18A9236D Sergey M.

[phihag]

I apologize; we should have done this right from the start. The certificate should be available at your keyserver of choice shortly.