Validation improved

Added a more flexible and convenient method to validate the form data.

You can add a validator with:
```objc
[row addValidator: [XLFormRegexValidator formRegexValidatorWithMsg:@"no valid email" regex:@"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}"]];
```
Writing a custom validator (have a look at XLFormRegexValidator):
1) Implement the ```XLFormValidatorProtocol```
2) override with your custom rule:
```objc
- (XLFormValidationStatus *) isValid: (XLFormRowDescriptor *) row;
```