Safely stop tests and build running on npm install

[pimterry]

This halves the number of tests we run, working towards #241.

This script makes NPM prepublish never run the tests or CS build. Instead, those are run in NPM@4's new prepublishOnly step, which only happens on an actual publish, not on simple npm install's.

To make this safe, we have to catch any users that aren't using NPM@4, but are trying to publish - require-NPM4-to-publish does that, and gets triggered by any actual publishes when they trigger prepublish.

Tested locally with NPM 4.0.3 and NPM 3.10.10.

[emirotin]

Great job!
NIT: I'd make npm4 lowercase

[pimterry]

Sure, done.