Flow types and maintenance fixes

Merge pull request #22 from hull/release/v2.0.1

Fix flow types

Release v2.0.0

• BREAKING HullClient is a set of ES6 classes now, not a Function anymore - this means you cannot do const hullClient = HullClient() anymore, you always need new keyword
• BREAKING as method is not available anymore, use asUser
• BREAKING utils.groupTraits method is not available anymore, use utils.traits.group
• BREAKING traits method second parameter accepting source and sync option is not available anymore. Sync calls are not available at all, and if you need to apply source to your traits you need to do it before passing payload to traits method
• BREAKING hullClient.api.get methods are not available anymore, use hullClient.get or hullClient.api(path, "get"), the same applies for all HTTP verbs
• the client now comes with full set of Flow types in src/types.js file

  import type { HullUserClaims } from "hull-client";
  
  const userClaims: HullUserClaims = {
    wrong_claim: "bar"
  };
  // this will throw flow check error since `wrong_claim` is not correct

• underlying HTTP library restler was replaced with superagent

Simplified babeljs configuration and documentation

• documentation split into API reference in API.md and getting started and "how-tos" guides in README.md
• cleanup babeljs configuration and use native NodeJS v6 with single babeljs plugin (transform-object-rest-spread) to allow object spread syntax (remove import/export syntax)
• run circleci jobs on all supported nodejs versions
• upgrade of dependencies