ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
Easier just to do:
```
if not isinstance(mask, int):
    raise ...
...
```
That way the remainder of the code is one line less indented, which is nice. Also, would be good to have an `assert_raises()` test check, just to keep our coverage up :)