a prototype with gulpjs

Make use of gulpjs for building. Available tasks:
* `hint`: for hinting all our js
* `test`: for testing coverage with `istanbul` and testing with `mocha`
* `build`: combine the above and also run `browserify` to create pouchdb-collate.js in `dist/` folder

Regarding backwards compatibility, `npm test` and `npm build` are calling the corresponding gulp tasks.