ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
ok, so I guess what I am doing is unmasking the bit representation and you are looking to mask it, that is, wherever the value is 1 in the mask, make it 0.

the equivalent should be np.bitwise(x, mask)