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)
hrm. hold the phone. we should add tests around the case sensitivity of the these keys. How that relates to sorting and whether there could be collisions.