Tag Inheritance

* subclass methods that don't specify tags, get the parent's tags
* subclass methods overwrite the parent's tags if the subclass specifies
  any
* subclasses can also do "sub foo : Tags(+new_tag -old_tag)" to add and
  remove tags.
* empty tag lists are supported now to completely override the parent's
  tags
Merge branch 'tags' of https://github.com/pboyd/test-class-moose into pboyd-tags
Bugfix: keys $... doesn't work on older Perls.