Option for trigger event, defaults to blur

Hello Paul,

Really useful directive. I've been using it for a bit, but I figured I should share this tiny modification to make blur configurable. For some fields where I have very specific patterns it is ideal not to wait for a blur event.

```html
<form name="userForm">
  <div class="form-group" show-errors="{ trigger: 'keypress' }">
    <input type="text" name="firstName" ng-model="firstName" ng-pattern="/^foo$/" ng-required />
  </div>
</form>
```

Thanks for the great directive!