Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
hmm, seeing this in practise I'm not sure `try` / `finally` makes sense.  nothing will use `__tagHelperBufferedStringValue` if an `Exception` is thrown within the `try` block.  and the scope manager will get messed up regardless since outer scopes won't be ended.  simplify this and use the same pseudo-nesting as is done for `__tagHelperScopeManager.Begin(...)` and `__tagHelperScopeManager.End()`.