Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
`throw` something, `Assert` a failure, or silently `parser.Optimizers.Add(parseTreeRewriter)` if you haven't updated the list.  probably need to do this either at the bottom of the `for` (e.g. `else if (i == parser.Optimizers.Count - 1) ...`) or add a `bool` variable to remember if you did something in the loop.