Update devDependencies to reflect our gruntfile.

We have a lot of packages in devDependencies that are actually
used by our build:production grunt process.

Update package.json to reflect this.
Add a heroku:development task to our Gruntfile.

This is required because we're using a forked buildpack for
Grunt support on Heroku.

See https://github.com/mbuchetics/heroku-buildpack-nodejs-grunt
Update README for Heroku.
Merge pull request #528 from alphagov/enhancement/build-process-for-heroku

Build process for Heroku