Rewrote tests with bats

As evidenced by the 11 prior commits, proper testing has revealed a
large number of gaps and inconsistencies in these scripts. bats is a
very capable replacement for the old test suite.

Moving gitstat into public repo

Added core README; dropped one not-so-useful script

Refactored code into multiple 'packages'. One day these may be
independently installable. For now it's just an organization method.

Rewrote main README and added per-package READMEs.

Also: bug fix on whiched and whichvi. They formerly propogated an error
if $EDITOR or $VISUAL were set to a non-existant executable. Also
improved the inline documentation.