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
In a real app, I would output all the minification/build stuff into a `dist` dir at the root of the project, which is the one that actually gets views in the browser. But in this case, we're tacking on this build stuff afterwards, which will break the entire tutorial, which is why opted to not go that route. Do you think we should refactor the tutorial to just use build tools from the start?