Use npm shrinkwrap

Lock module dependencies for consistent installs with
[shrinkwrap](https://www.npmjs.org/doc/cli/npm-shrinkwrap.html).

Add ignore file for node_modules/ as the dependencies are shrinkwrapped.

Shrinkwrap created with `npm shrinkwrap --dev` given the nature of the
project.
Merge pull request #3 from uhhs/npm-shrinkwrap

Use npm shrinkwrap