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
@bajtos Spoke with @altsang regarding the relevancy of build systems in this tutorial. His comments were:
- we should keep the example simple
- put the note in at the end saying devs should consider bundling and minifaction as part of the bundilng/build process
- it's advanced on lb functionality, not on completeness of dev
- we don't need to explain best practices for build/bundling

That's the short cut and paste of the conversation. We may add build automation into `loopback-getting-started-advanced` in the future, but that's not coming up soon anyways.

Closing.