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!
Merge pull request #103 from cballou/patch-1

Modified `FormValidator.prototype._validateField()` to support allowing custom messages.