Adding TypeScript definitions files to steem-js

Hi, it would be awesome if we could add typings to steem-js so it's well usable with TypeScript and has static type checking, refactoring support and tooling support (like autocompletion etc).

Therefore it would need to be added to https://github.com/DefinitelyTyped/DefinitelyTyped

And then it could be instaledl via npm install @types/steem-js

If I find the time I'll start doing it myself. But until that time comes, maybe somebody wants to start doing it already :)