socks5-server licensed under GPL3

[jgraef]

The crate socks5-server is licensed under GPL3, which as far as I understand, requires our software to be licensed under GPL3 as well. I don't want this, so we might need to write our own socks server implementation or find another crate.

• [ ] socks4 client
• [x] socks4 server
• [ ] socks5 client
• [x] socks5 server
• [ ] username/password auth. the subnegotiation isn't implemented. we need this.
• [ ] handle bind/accept, associate properly (we don't need this, but a full socks5 impl would be nice)
• [ ] gssapi auth (we don't need this, but the RFC states that a proper socks implementation must support this)

[jgraef]

I wrote a basic socks4a/5 server implementation, and removed the socks5_server dependency.

[jgraef]

For reference:

• socks4
• socks5

[jgraef]

Since we don't depend on socks5_server anymore, I moved the todo list to a new issue (#24).