Added support for custom messaging on a per field/rule basis.

Modified `FormValidator.prototype._validateField` to support allowing custom messaging on a per field/rule basis. For instance, you can now use the following syntax to target applying a unique validation error messaging to the field `fieldName` when it fails `required`:

```js
validator.setMessage('fieldName.required', 'This field is required, yo');
```

The format is `fieldName`.`validationRule` when using `setMessage()`.

Hope others find this helpful!
Merge branch 'patch-1' of github.com:cballou/validate.js into patch-1