Method to unmask and remove the keyup event of the element.

I needed a method to reset the mask and events in a field. Now is possible to use the "unmask ()" method and then apply any mask's pattern again.

### Example of usage:

```javascript
var el = document.querySelector(".mask.phone");
VMasker(el).unMask();
```

In my specific case, the problem was with the numbers of Brazilian cell phones, which have a ninth digit in some cities as São Paulo. I needed to apply mask patterns according to the number of typed characters. Below my solution using the new method.

HTML markup 
```html
<input type="text" name="phone" id="phone" class="mask phone">
```
Javascript code
```javascript
  //applies default mask pattern
  VMasker(document.querySelector(".mask.phone")).maskPattern("(99) 9999-9999");            

  //sets keyup's event to check which mask pattern will be used
  var phones = Array.prototype.slice.call(document.querySelectorAll('.mask.phone'));
  phones.forEach(function(el, i) {
      addEvent(el, 'keyup', function(){
          var phone = el.value,
          digits = phone.replace(/\D/g, '');

         // removes the last mask pattern and keyup event setted by maskPattern method
          VMasker(el).unMask();

         // verify length of digits and apply correct mask pattern
          if(digits.length > 10){
              VMasker(el).maskPattern("(99) 99999-9999");
          }else{
              VMasker(el).maskPattern("(99) 9999-9999");
          }
      });
  });
```

### Note:
I changed the way it was applied the keyup event to "DOM Level 0 property" only to override of the event "keyup" outside the function's scope "bindElementToMask" with an easier way. :D