[Design] Add TagHelperDescriptorResolver implementation.

- Added a basic TagHelperDescriptor creation mechanism to the TagHelperDescriptorResolver.
- Added an ITagHelperTypeResolver to be responsible for determining valid TagHelper types.

#99
please read the Engineering Guidelines:
> Unit test class naming

> Test class names end with “Test” and live in the same namespace as the class being tested. For example, the unit tests for the Microsoft.Fruit.Banana class would be in a Microsoft.Fruit.BananaTest class in the test assembly.