Added git-status.sh and configure its use in gitprompt.sh.  Updated README.

Hello, I added `gitstatus.sh` as a functional replacement of `gitstatus.py`.  The README was updated, and `gitprompt.sh` was modified to prefer using `gitstatus.sh` (if it exists) over `gitstatus.py`, include the color cyan, and apply cyan to untracked files.