Add TypeScript declaration, use ES6 syntax and ignore .travis.yml

[hossam-magdy]

@STRML: could you check this one?... it is confirmed that all the tests are passing!

Changes introduced:

• support TS, add TypeScript declaration file: index.d.ts
• refactor using ES6 syntax (Class) and add JSDoc
• include only necessary files in pkg… ignore unnecessary included files like:
  • .travis.yml
  • .nycrc
  • .eslintignore

And to avoid such changes being stale like https://github.com/STRML/async-limiter/pull/6, I tried to keep the code changes as minimal as possible, like even avoiding class properties/fields.