Auto-run Bower during npm install & add README development guide

By defining npm scripts within our package.json, we gain the ability to
install bower packages and run grunt without having to globally install
either tool. This technique is inspired by jQuery's own package.json,
which uses the same pattern.
Reinstate description-to-come disclaimer within README

Adding a cross-link to WP-API provides some context, but while I provided
installation instructions I did not elaborate on what the client-js repo is
and how it's used, so I should not have removed the 'description to come'
disclaimer text.
Merge pull request #10 from kadamwhite/simplify-installation-and-update-readme

Auto-run Bower during npm install, and add README development guide