Gruntfile.js: Add swagger-js-codegen task

And add that task to 'build'.  The dependency in package.json assumes
semantic versioning, but I've only actually tested the task with:

  grunt-swagger-js-codegen@0.2.13 node_modules/grunt-swagger-js-codegen
  |-- q@1.1.2
  |-- lodash@2.4.1
  |-- request@2.49.0 (caseless@0.8.0, json-stringify-safe@5.0.0,
  |     aws-sign2@0.5.0, forever-agent@0.5.2, stringstream@0.0.4,
  |     oauth-sign@0.5.0, tunnel-agent@0.4.0, node-uuid@1.4.1, qs@2.3.3,
  |     mime-types@1.0.2, combined-stream@0.0.7, form-data@0.1.4, bl@0.9.3,
  |     tough-cookie@0.12.1, http-signature@0.10.0, hawk@1.1.1)
  `-- swagger-js-codegen@0.2.6 (mustache@0.8.2, js-beautify@1.5.4,
        jshint@2.5.10)

I've added the Swagger API spec as a submodule for easy reference.
README.md: Use reference-style links and mention Compass

The reference-style links make the text itself less cluttered, and
protect the text from future changes in the referenced URLs.