[ENH] NiftiMasker: added time mask

tries to address #165 
> I think that the name is bad.
+1

> Avoiding the bug we had in the anova haxby example :).
The bug was mainly due to the fact that the shape of the session mask was not checked. I think that the main advantage is to be able to apply a condition mask to the data without having to load them manually (and still delegate that to the NiftiMasker).

However, given that this is only used for transform, shouldn't it be a parameter for the transform function rather than for the Masker in general?