ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
The problem is you just want to force them to be zero and the np.bitwise commands try to be cleverer than that. I don't know what the 'b's refer to in your example, but basically, to take mask 3 example
5      -> 4
101  -> 001

6      -> 4
011  -> 001

7    -> 4
111->001