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!
here you deleted some code that was released in v0.8. If somebody updates MNE-Python his code will break if he was ussing this function. So in v0.9 we keep this function that will display a warning saying that it will be removed in v0.10 and that one should use the object. ok? see an example 

https://github.com/mne-tools/mne-python/blob/master/mne/report.py#L697