handle inverted z axis when plunging

This includes some minor fixes to related, and seemingly
out of date tests that include z coord, and then a rapid

(which I'm assuming is the retract feature)
cover cases in #layer
Merge pull request #9 from tmpvar/invertedZAxis

allow for inverted Z axis
Bump npm version
Whoops. Forgot to commit the build.
Remove --dev from build.