Add type definitions

[Evanlec]

For those of us using Typescript or similar, it would be nice if there were some type definitions for this library.

I looked for a types package in NPM under the standard @types namespace but it appears none has been created:

$ npm install -D @types/fuel-soap
> npm ERR! code E404
> npm ERR! 404 Not Found: @types/fuel-soap@latest
> ...