Feature/add gulp example

- Generate `lb-services.js` via `gulp-loopback-sdk-angular`
- Rebuild `lb-services.js` on changes in watch mode
- Minify javascript files
- Bundle javascript files into `bundle.js`
- Reminify and rebundle on changes in watch mode
> don't minify javascript at all.

At the end of the tutorial, you can add a note that developers should consider bundling & minification of the javascript sources and point them to possible solutions.

Keep the tutorial focused on angular-loopback integration and leave out the details that are too tedious and not related to the topic.