[ENH] NiftiMasker: added time mask

tries to address #165 
> 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?

Possible. The real argument is that it's data dependent (ie has a shape
in n_samples) thus giving it at build-time break cross validation.

Which tells us that sessions shouldn't be given at build time :$