ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
And again, I don't think it's actually necessary to manually broadcast this to the proper shape:
```
>>> np.bitwise_and(np.array([1, 2, 3, 4, 5]), 5)
array([1, 0, 1, 4, 5])
```