Finalize TarSum Version 1 w/ refactor

The current Dev version of TarSum includes hashing of extended
file attributes and omits inclusion of modified time headers.

I refactored the logic around the version differences to make it
more clear that the difference between versions is in how tar
headers are selected and ordered.

TarSum Version 1 is now declared with the new Dev version continuing
to track it.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Well, there is an interface `tarHeaderSelector`. I made it so that the `tarHeaderSelectFunc` type implements the interface. It's like `http.Handler` and `http.HandlerFunc`. The version numbers are in the func name because I hadn't thought of a better way to differentiate them.