Enable tar diffing

@bfulton 

This is where I am currently with tar-diffing.

I made a test file that does the most basic flow possible.  It requires the swipely image imported (the post-build image), and the base image it is built from.  All files are in tar format, not tar.gz.

This current implementation, despite being insanely faster than what we were seeing yesterday is still incredibly slow in comparison to last weeks.