[FEATURE] Add Wallet class that generates wallets.

Wallet.generate() computes a new random wallet.
I don't think we should use `const` in ripple-lib. In any of our JS stuff that never touches the browser, `const` is fine. In ripple-lib it should be avoided. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const#Browser_compatibility