Play a friend feature

Hi @EmilS,

First of all, thanks for your **great** 2048 multiplayer version! I'm using it a lot and having much fun with colleagues :)

I'm implementing a "Play a friend" feature, much needed for us (currently, we savagely refresh browsers to try to find against each others..) on your code. Here is my first commit.
Here, basically, what I have done:

- Now users connect to sockJS on dom loaded, they trigger different events:
  - 'register' -> register a their user-friendly 4 digits hash on the server, under their io connexion
  - 'play-friend' -> send a 4 digits hash of a friend, to launch a game against him
  - 'find-opponent' -> find a random opponent waiting too, the current feature
- I reordered some code parts into function to be able to mutualize them in random mode or find friend mode.
- I showed the stats all time, I find it quite fun. I distinguish now players on plateform, and waiters that await for a random match

This is working for me right now.

Now, what I'd like if possible:
- your feedback on that (the principle)
- eventually clone my work and test it, and have your feedback
- if you like it, what should be done on this branch before you merge it

Best :)