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 for signed-in users.
* 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 (using simple_form and Ken's suggestions on how to populate the drop-down SELECT box).
* Updating the "show.html.erb" view to show the email addresses of both players in the game above the initialised board.

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? 

Improvements needed:

* Currently you can select to play against yourself. May want to prevent this.
* There's also a blank entry in the drop-down box.
* Can't figure out how to override the simple_form label for "player_2" (so that we can change it to something more user-friendly like, say, "Opponent".)
* CSS formatting needs improving.
* We may want to create an association between User and Game, so that we can streamline some of the code (for fetching the user based on game.user_id). Not sure if we can do the same to fetch the user based on game.player_2?