make the player a prefab, and allow 2 to be controlled

[nathanleiby]

I refactored the input manager a bit, so that when you have a controller and a keyboard, it will auto assign a role to each player. Here are two players being controlled at once, with the controller being assigned dynamically:

In the 2nd commit, I added a PlayerInputManager that allows players to join when you press a key/button

[nathanleiby]

Was hard to rebase the unity metadata files, will just be easier to make a new PR