ENH: Add support for cross generalization for time_gen

@dengemann @agramfort

This is work in progress, I will let you know when to review it.

Thanks!
I would say:

```
gat.fit(epochs, y=y)
gat.predict(epochs_new_or_old)
gat.score(epochs_new_or_old, y=y_new_or_old)
```

as the sklearn API