WIP/API/ENH: create epochs in ICA space

Here I propose two new convenience methods for the ICA class which
allow users to directly create epochs in ICA space without previous
export to Raw. This addresses #319.
@agramfort the API naming may sound a bit cumbersome / verbose but I don't see any other solution unless we break with our overall API. I would prefer the solution proposed to things like `ica.sources_as_epochs1/2`. Also, I don't see any reason to only support this for raw, as the solution in the Epochs case is really straight forward and it would be nice to haven (e.g. identify evoked sources, etc.).
If you're happy with this, I will put it in practice as proposed.