Remove gulp utilities

[stevenle]

We initially added a few gulp tasks with browserify to make it easy to get started with airkit but it's not easy to maintain that sort of dependency since the javascript build ecosystem changes so fast year over year. Instead, we should provide example docs on how to set up airkit using common systems like webpack, etc., or just let new projects figure out how to integrate airkit into their systems.

At the minimum, we just need to be able to import airkit modules using ES6 module syntax, e.g. import {Modal} from 'airkit/modal'. Build systems can transpile ES6 module syntax to AMD or CommonJS or whatever.