Add support for username password authentication

[guylabs]

Currently the library only supports NO_AUTH authentication as seen here:

java-socks-proxy-server/src/main/java/org/bbottema/javasocksproxyserver/Socks5Impl.java

Line 132 in fc68e1e

return ((Methods.indexOf("-0-") != -1) || (Methods.indexOf("-00-") != -1));

The USER_PASSW authentication with value 2 is not supported.

This would allow to use the library for authentication tests.

[bbottema]

That would be a nice feature to add. Unfortunately I know next to nothing about the actual inner SOCKS implementation details. I will gladly review and accept pull requests for this.