Fix unittest

Help text specified wrong option.

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.

Specify full path to POSIX utilities; Clean up scripts' output

Added --working-directory to unittest; bug fixes

Initial commit of cleaned-up dev branch