De-couple bundling from Punchcard

[sam-goodwin]

As of #53, Punchcard's runtime dependencies interact with webpack directly to create an efficient bundle for AWS lambda. This coupling pollutes the runtime dependencies and is way too opinionated - developers should be able to use any tool to create their bundle.

We should move the webpack packaging out of Punchcard and instead have developers set up their repo to generate and put a bundle in a known location. I.e. use webpack.config.js and webpack as a devDependency.