Addressed pranav's code review comments.
Add FormTagHelper.

- Added the FormTagHelper.
- Utilized the IHtmlGenerator to share base functionality with the HTMLHelper counterparts.

#1246
Add tests for FormTagHelper

- Modified FormTagHelper to utilize the AntiForgeryToken when the Action is URL based (different behavior).
Addressed code review comments.
Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
Add AnchorTagHelper tests.

- Fixed a typo in resource string.
Addressed code review comments.
Addressed code review comments.
Add ValidationMessage TagHelper.

#1250
Add ValidationMessageTagHelper tests.
Addressed code review comments.