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, I guess I see how you can use the _and function, but I cant see yet how to get the mask right

It needs to be this, but I can't think of a smart way to get there.
int('001111111',2)