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 just looked up how to do what I believe the MNE-C (and what I meant) option does it:
``bin_ev_1 = np.binary_repr(events[:, 1],50)``

Note this doesn't work need to dig in why (something about np.binary_repr only working on one number) and the fifty should change and be made flexible to whatever the max event_id binary size is.

Then reset the value of each string in the ignore value to be 0
```
bin_mask = np.binary_repr(mask,50)
zero_indices =  for v in range(len(bin_mask)):
    if bin_mask[v] is '1':
        index.append(v)
```
This index can then be used to manually set those values to 0 in events[:,1] and events[:,2]

Does that make sense?