added play-scala.js showcase

I've created a few example using Play and Scala.js and I think it will be useful for everyone
who is learning both Play and Scala.js like me. Some of the examples, I modified from others
like TodoMVC. I modified to make it works with Play backend. I also created a few more
like HTML5 file upload, Hangman, and WebSocket Chat (I took the code from the old Play 2 example and
modified to make it works with latest version of Play and Scala.js)
I hope this project will be practical examples and make people who are learning love Scala.js like I do.

Thank you everyone for creating this great work.