Added focus as a trigger event for mask

When using mask with an input field that uses ng-required, if form validation fails the mask field is automatically focused but mask's eventHandler isn't triggered, which causes the first character typed to be behind the caret:
```
^ = caret
Field: [        ]
on validation failure because of empty && required:
Field: [^       ]
after typing one character, "G":
Field: [^G      ]
after typing another character, "O":
Field: [O^G     ]
```

This seems to be an issue on webkit browsers supporting the required attribute.  Fixed by binding onfocus to the eventHandler as well.