Feature: Add Unix domain socket (UDS) support to Server with unix:// URI scheme and add advanced UnixServer class. (#120 by @andig)

// new: Server now supports "unix://" scheme
$server = new Server('unix:///tmp/server.sock', $loop);

// new: advanced usage
$server = new UnixServer('/tmp/server.sock', $loop);

Restructure examples to ease getting started (#136 by @clue)

Improve test suite by adding forward compatibility with PHPUnit 6 and ignore Mac OS X test failures for now until Travis tests work again (#133 by @gabriel-caruso and #134 by @clue)