Refactor ParseTag

* refactor ParseTag, removing the hint. The hint was only used by code when it expected a specific tag type.
* introduce type specific Tag parsers, these replace all the uses of ParseTag with hint and return a specific Tag implementation, not an interface, so the Tag's implemenations values are available without calling a helper method
* Add unit tests for new methods
Merge branch '106-parsetag-refactor' of https://github.com/davecheney/names into 106-parsetag-refactor