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
// initiate validator with single required rule
var validator = new FormValidator(
    'form-identifier',
        { name: 'fieldName', display: 'your field name', rules:
	'required' },
	    function(errors, evt) { }
	    );

	    // set custom error message display for fieldName input
	    // field and required rule
	    validator.setMessage('fieldName.required', 'This field is
	    required, yo');
	    ```

	    When using `setMessage()`, the newly supported format is
	    `fieldName`.`validationRule`. This allows you to uniquely
	    target individual field/rule combos for error messaging.
	    Very useful if you want to give more instruction/detail to a
	    user.

	    This scratches my itch for overriding messaging on a low
	    level. Hope others find this helpful!

validate.js

@@ -299,7 +299,7 @@
 
             if (failed) {
                 // Make sure we have a message for this rule
-                var source = this.messages[method] || defaults.messages[method],
+                var source = this.messages[field.name + '.' + method] || this.messages[method] || defaults.messages[method],
                     message = 'An error has occurred with the ' + field.display + ' field.';
 
                 if (source) {