Add custom validator

Added a local validator `custom`.  It takes only one option, a string naming the function it will run on validation.

```Javascript

App.Task.reopen({
  validations: {
    start: {
      custom: 'validateStartIsBeforeStop'
    }
  }

  validateStartIsBeforeStop: function() {
    if ( this.get('start') > this.get('stop') ) {
      return 'Start must be before stop';
    }
  }
});
```

I don't know if this is the ideal way to define custom validators but it gets the job done.  I hope this or something like it makes the cut.