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!
>
> Ok, I'll put an underscore for each stored values. However, I'm not sure
> to see the logic. e.g. epochs have an array 'times' not 'times_' .
>
sklearn uses _ to differentiate attributes which are data dependent vs
parameters. Not all attributes have _. Just the ones added by fit method