Modify parser to group html begin/end elements.

- Added a "Tag" block type.
- Wrapped all begin/end elements in a "Tag" Markup block.
- Attempted to change as little as possible in the parser since it is easily the most fragile part of Razor.

#75

Since this impacted a large amount of the existing tests I've created a separate "test" PR.  I felt that the existing tests did more than enough verification for this "feature" since it's at the core of the Razor parser, let me know if you have a different opinion.

Test PR: https://github.com/aspnet/Razor/pull/87
No concurrency involved.  More or less just figured i'd make it concurrency proof so it works in every situation.  The DisposableAction class is a utility class so if we ever use it in a concurrency sensitive piece of the Razor parser it may come into effect?  I'll keep in synchronous for now.