Add an npm package build script and remove README references to Bower

Several comments in the WP-API/client-js repo from which this project's README was derived do not fully apply to angular-wp-api: There's no need to use Bower in this case, and the "A Note on Grunt" section was referring to steps which did not exist in this repository.

This PR adds a build script (making the Note on Grunt section and several other comments more accurate), and updates the README to remove references to Bower. Grunt will now automatically run on `npm install`, through a custom postinstall NPM package script command; Grunt is now also aliased to `npm run build`.

Awesome work on this so far, I can't wait to see where it goes!