Add HTTP/TLS obfuscating. [SIP001] (shadowsocks#1009)

Add experimental HTTP/TLS obfuscating as an **optional extension** of shadowsocks protocol.

More discussions can be found here: shadowsocks/shadowsocks-org#26

As this feature is still a SIP (Shadowsocks Improvement Proposal), it's very unstable and experimental. So, 

1. Don't enable it unless you know what it is.
2. Be very careful when using it in production environment.

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Fix a typo

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Fix shadowsocks#656