add exclude functionality

* Exclude dirs listed in the $_Z_EXCLUDE_DIRS array
docs

README
also removes zsh backward incompatibility warning
revert an accidentally commited bit
Change aging algorithm.

Aging happens when the sum of rankings passes a threshold. Raise this
threshold from 1000 to 6000.
Change aging multiplier from .9 to .99.

This formula is still pretty naive, and wants improvement, but seems to
be nicer. The previous constants tended to forget things too fast.

Closes #53, for now.
You only need one '=' character to check if something is equal
Merge pull request #76 from nmeum/master

You only need one '=' character to check if something is equal. closes #75 and #76.
reflect aging formula changes in man page and README
More groff_man-ish coding style
Merge pull request #78 from ntsuji/groff_man

More groff_man-ish coding style
Add warning message if z.sh is executed instead of sourced.

Closes #74
Add Makefile for README
Use _Z_CMD in z -h
-c option restricts matches to subdirs of cwd.

Closes #83
Make -c without args work like regular z does
test directly, instead of testing exit variable
Allow options to be combined e.g -clt
update README to match man page
Remove extra . from man page.
Be explicit that -c matches should start with cwd, not just contain it.
chmod -x z.sh