Added recommended attributes to elphyio spiketrain

Hi,
just added the possibility to read t_start, waveforms and sampling_rate from elphy files in neo format.
I tested it against the old files and a new one, passing the baseio tests.

Thanks for considering the modifications!
This seems to reverse 1a5cb0593d8f1b26, is that intentional? Probably a good example why regression tests are useful...