Widget Button Toolbar help and contribute buttons optional

- HTML template updated to use `ng-if` to either create or remove the Help and Contribute buttons based on the values provided in the attributes
- Directive updated to apply values to `$scope.helpRef` and `$scope.contributeRef` and check the existence of the attribute values provided
- Removed a useless e2e test line of code
- Updated demo example to mention the Help and Contribute buttons being optional
Merge pull request #148 from stulees/chore/widget-button-toolbar-optional

Widget Button Toolbar help and contribute buttons optional