Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
nits:
- `GenerateTagStart()` -> `GenerateStartTag()`
- `GenerateTagContent()` -> `GenerateContent()` or `GenerateElementContent()` since the only tags are the start and end
- `GenerateTagEnd()` -> `GenerateEndTag()`