Add @addtaghelper directive. 

- Also added some infrastructure pieces for it such as the ITagHelperDescriptorResolver and the default implementation TagHelperDescriptorResolver which will be filled out in a later commit.
- Reworked some extensibility points to allow accessibility of the descriptor resolvers per offline discussions.
- Fixed existing tests to work with new RazorParser.
- Validated directive syntax tree creation, errors and code generation.

#111
You can already do this by replacing the `TagHelperDescriptorResolver`... Not the cleanest but possible.  We do plan to add a first class way to add tag helpers via code without a directive but that's another issue: https://github.com/aspnet/Razor/issues/162