Checkout master before publishing

Jenkins checks out a single commit, which you can't push changes
from.
Add npm-debug.log to gitignore
Don't use the files array in package.json

This whitelists files to include in the package, which is not useful
when we want to include all of the assets.

Instead, use .npmignore to stop the publish script from being included.
Merge pull request #8 from alphagov/npm-publish-tweaks

Tweaks for publishing to npm