Add TypeScript types

[pahuta]

Add corresponding '.d.ts' file to make possible using package with Node.js server written in TypeScript.

[ToshieUya]

Would love to have this.

[adohertyTS]

Hi all! I realize I'm responding rather late here but thank you for the suggestion, I know TypeScript is quite popular. Our product team will consider this feature request.

[Smeagolworms4]

Add basic typing hear : #19