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
> Do you think we should refactor the tutorial to just use build tools from the start?

Either that, or don't minify javascript at all.

It would be best to ask @seanbrookes and @anthonyettinger about their opinion too, they have more knowledge about the best practices for this.