FIX/ENH: loading EDF annotations

I made sure that the stim channel does not get resampled, and that the int16 values (each containing two characters) are not clamped.

Parsing the stim channel to get events of the form [onset, duration, 'annotation'] remains to be done.

I'm not sure where to store these events, though. Currently I consider not to store the events at all. Instead, a member function would simply parse and return the event list. Any thoughts?