ADD digital mask option

Addresses https://github.com/mne-tools/mne-python/issues/1207. Adds digital mask option to `read_events` and `find_events`.
got it, so this is what I got.
you take the bitwise_not of the mask and then you do the bitwise_AND, essentially you get this.

```
    e.g.      7 '0000111' <- trigger value
             37 '0100101' <- mask
         ----------------
              2 '0000010'
```