ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
I think `mask=0` is the same as no mask, right? In that case, I would do `mask=0` by default. You can then make the check `if mask != 0` here for efficiency, or remove it completely for simplicity. WDYT?