Find the targets store?

[noamross]

Right now tar_read_version() requires a store argument if the targets store is not the default. Should targets attempt to find the targets store in the project by heuristics? (Should contain meta/meta, meta being of the right form.)

This is not unlike looking up the commit from the tag or other mutable ref. It could return the path that it found as a message, so that in the future a definitive store could be used (per #7)