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 think something low level of this sort would be useful in `classifier.py` if it can be used in a pipeline. e.g.,

```py
filt = mne.decoding.FilterEstimator(info, 1, 40)
scaler = sklearn.preprocessing.StandardScaler()
concatenator = mne.decoding.ConcatenateChannels()
time_slicer = mne.decoding.TimeSlicer(step=20 ms)
clf = Pipeline([('filter', filt), ('slicer', time_slicer), ('concat', concatenator), ('scaler', scaler), ('svm', clf)])
```
@agramfort wdyt?