Feature: Support SOCKS over TLS (sockss:// URI scheme) (#70 and #71 by @clue)

// new: now supports SOCKS over TLS
$client = new Client('socks5s://localhost', $connector);

Feature: Support communication over Unix domain sockets (UDS) (#69 by @clue)

// new: now supports SOCKS over Unix domain sockets (UDS)
$client = new Client('socks5+unix:///tmp/proxy.sock', $connector);

Improve test suite by adding forward compatibility with PHPUnit 6 (#68 by @clue)