Change name in package.json

This commit makes it clear that *this* package.json is not the
package that ends up on the registry at
https://www.npmjs.org/package/govuk_frontend_toolkit
Bump frontend toolkit to 3.0.0
Merge pull request #144 from alphagov/publish-npm-from-this-repo

Change name in package.json and bump to 3.0.0