Add typescript support

[SolcAndMe]

No description provided.

[0xTARC]

For bundling the SDK into a typescript package, looks like mycelium pools sdk uses this command for bundling their typescript package:
tsc && babel src --extensions \".ts\" --out-dir ./dist

link to repo: https://github.com/mycelium-ethereum/pools-js/blob/master/package.json

Might also need to change a couple tsconfig settings. This guide looks nice & minimal for configuring that: https://cameronnokes.com/blog/the-30-second-guide-to-publishing-a-typescript-package-to-npm/ . Could also probably copy the pools-js tsconfig as well

[0xTARC]

Work started here: #12

Still needs:

• Changing src/index.js to .ts file (with passing tests)
• Changing test/test.js to ts file
• Test the index.ts file directly instead of testing the generated js. (imo superior, open to other opinions though)
• Add types to sdk functions (src/index.ts)