Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
thought for the future: can all tag helpers except those with `ContentBehavior.Modify` (or `ContentBehavior.None` and combined with at least one `Modify` helper) be reused?  that is, could these declarations also call `CreateTagHelper<THelper>()`?  (we don't really know how expensive replacement `CreateTagHelper<THelper>()` methods will be)