Big rewrite.

All the complex comment and code analyzing is now done by Merger class,
the doc_comment_parser now just parses input into a linear array of at-tags.
The Merger completely replaces DocComment class and most of the complex
work in DocCommentParser.

Most of the tests are now also at top level tc_jsduck - so that we
mainly test the whole jsduck module, allowing the internal classes to be
more easily reorganized.

@ignore and @hide are now simply aliases of @private.