[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.

Also, as an exercise I replicated the MVC behavior in this class (and then removed the code because it's not relevant to this PR), my point is that we can eventually flesh out the framework pieces of the **Microsoft.AspNet.Razor.Runtime.Test** project to mimic a significant portion of MVC's runtime with how its built.