rename z() to _z(), aliased to z. update manpage and readme.

Alias allows cmd to be overriden, and makes it so z prints to STDOUT as
expected. Not well tested on zsh.
Use actual return status, not 'local' builtin's.
Check return status of awk when adding an entry.

This will avoid truncating data file to zero length when disk is full.
Merge pull request #30 from Enlik/master

pull request: use actual return status; don't truncate file when disk full
minor style change after pulling from Enlik
we don't care if awk reports a failure due to no .z file. Closes #31
better error handling for issues 30 and 31
add info about how to install the man page as mentioned in #gh-32
only cd to completions if we've actually tab completed them

We decide you've tab completed if the last argument begins with /, and is an
existing directory. This may still cause unexpected behavior if you do that
without tab completing.
add tab completion section to man page
fix zsh tab completion, which I believe has been broken since 84ace8e8a93dff216dda
remove a debug line
use a case statement rather than a bash substring, for portability
documentation
documentation
documentation
stop rewriting the db file when z is used, remove useless code from
common()
revert changes to common pending further thought
documentation
another go at removing redundant code from common