Refactor ActionTag to expose a UnitTag() method.
good rename suggestion from Horacio
make the use of t.id consistent
no need to export internal attributes
Refactor per @thumper.  One key difference from his suggestion is that I have exported names.ParseActionId, because it's still useful outside of the names package
Don't export ParseActionId
Expose Sequence
Merge pull request #11 from johnweldon/action-unittag

Refactor ActionTag to expose a UnitTag() method.  
Also restructure internals of ActionTag to be more useful.