WIP ENH: return last n samples as epoch

closes https://github.com/mne-tools/mne-python/issues/1474

the current `RtClient` + `RtEpochs` API doesn't allow querying continuous data and using them for analysis. Together with @LauriParkkonen, we figured that the easiest way to make this work (with the FieldTrip connector) was a function that returns, say the last n samples as an epochs object. This would allow using it for the rest of MNE-Python computations.
can you raise an explicit RuntimeError exception?