[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 class isn't meant to be used directly by users unless they're doing some pretty crazy customization's of tag helpers so I'm leaving this undocumented.