ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
Sure,
mask is 128
max event_id is 511
``` 
mask = 128
bin_length = len(np.binary_repr(np.max(event_id[:,2])))
bin_mask = np.binary_repr(mask,bin_length)
index = list()
for v in range(len(bin_mask)):
    if bin_mask[v] is '1':
        index.append(v)
```
Then you find that the index = 1
Then you change all the binary representations of events[:,1] and events[:,2] to be value 0 at index 1
Then convert them back to ints. 

The result is any value over 128 (gets reduced by 128) i.e.
129 -> 1
511 -> 383
1     -> 1
5     -> 5