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
break name up a bit with underscores e.g. `CSharpCodeGenerator_CorrectlyGeneratesMappings_ForAddTagHelperDirective()`
(nits: could be "GeneratesCorrectMappings" and could leave out "For")