Added create new game functionality

OK I've had a go at setting up a minimal form of the "create new game" functionality. Changes include:

* Updating the header to reflect when you're logged in, and creating a new "create game" link.
* Creating a "new" method in the Games controller and "new.html.erb" view which lets you select another player in the database.
* Writing a "create" method in the Games controller to save the new game into the database, assign both players to that new game, and redirect the user to the show view.
* Updating the "show.html.erb" view to show the email addresses of both players in the game.

It seems to work okay on my laptop, but it'd be great if someone could review this and test it to make sure it works for them. 

Quick side question: I noticed that the Game model has a "title" field - I didn't put anything in this when I created a new game entry - what's this used for?