Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
looks incorrect for a self-closing `<input/>` tag. not a fan of `GenerateTagStart()` actually generating a self-closing tag and `GenerateTagEnd()` being a no-op in this case.  the issue is in part naming e.g. `GenerateStartOrSelfCosingTag()` and `GenerateEndTagIfNotSelfClosing()`.
but would prefer self-closing to be a _declared_ trait of a tag helper.  `TagHelperDescriptor` would include this information as well.  generated code would call `GenerateStartTag()`, `GenerateContent(), and `GenerateEndTag()` versus `GenerateVoidElement()` (or `GenerateSelfClosingTag()` or `GenerateEmptyElement()` to be more XMLish and less HTMLish) depending on this setting.
if we need this to be a runtime decision up front for some reason (I don't see that immediate requirement), property should instead surface in `TagHelperExecutionContext` and generated code would understand values and call the appropriate methods.