Remove Year Model

Hey @Bigspencey, here's my first contribution: I removed the Year model and made year an attribute of Season instead. It struck me as unnecessary to have a separate model to keep track of years, and it created a complicated set of associations. 

Removing Year also required some changes to Season.populate_win_loss; I just tested it and it seems to work. We should develop some proper unit tests for that method though, given how important it is.

Let me know what you think!