Add example for Queue

example/queue.rb

@@ -0,0 +1,16 @@
+#require 'mruby-thread'
+
+q = Queue.new
+th = Thread.new(q) do |q|
+  q.push 1
+  q.push 2
+  q.push 3
+  nil
+end
+while true
+  Thread.sleep 1
+  v = q.shift
+  break unless v
+  puts v
+end
+th.join