Allow for functions when overriding default validator messages 

When overriding default validator messages it should be possible to use functions : 

```
_.extend(Backbone.Validation.messages, { 
  required: -> I18n.t('activerecord.errors.messages.blank')
})
```

similar to how it's done for validator message inside the model. Without this fix we're getting error : 

```Uncaught TypeError: Object function () { ...} has no method 'replace'```