Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
another possibility is for parser to remember original element in the Razor source was void.  the various chunks related to tag helpers and start tags would preserve this information for code generation.  and code generation would do the right thing.
while this is nice in that it leaves it up to the user to decide whether to follow the rules for HTML void elements, the approach constrains what users and tag helpers can do.  for example users must write `<select for="Strings" ></select>` though `<select for="Strings" />` seems more natural for the targeted tag helper -- a `<select>` helper would likely have `ContentBehavior.Replace`.  similarly a tag helper couldn't conditionally result in `<hr/>` or `<div class="horizontal-rule"></div>`.
not sure of the first version requirements here.  this would work for MVC's tag helpers but...  /cc @DamianEdwards 

note: `<input type="checked" .../>` would actually generate _two_ void elements :smile_cat: