[Design] Add TagHelperManager for TagHelper runtime.

- The TagHelperManager is injected into the RazorPage and is used to render Tag Helpers.  Specific examples can be seen in this PR: https://github.com/aspnet/Razor/pull/135
- Tested out the stack based tracking of tag helpers and their corresponding attributes.

#130
This is the same format that the existing **Microsoft.AspNet.Razor.Test** project uses: putting reusable pieces of code for testing purposes under the framework namespace/folder.