Refactored directive version of URL Field

- Simplified directive and HTML template to better leverage Angular api
- Updated with self validating logic and providing an invalid message
- translation.json updated with invalid message
- Optional ngModel to apply $setValidity from parent controller
Merge pull request #119 from stulees/feature/url-field-validation

Refactored directive version of URL Field
[chore] Integrate widget-tester tasks