Implement .d.ts type definitions

[trongthanh]

Let's write some .d.ts type definition file to leverage auto-completion from VSCode and other editor that use TypeScript as auto completion back end.

Note:

• How to write .d.ts file: http://www.typescriptlang.org/docs/handbook/declaration-files/templates.html
• How to include definition file with node module: http://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html

[trongthanh]

Implemented in #11