Light-Client API for JavaScript

At present this just exposes the Tendermint Light Client's verification logic via WASM. This allows simple access to verification from JavaScript:

import * as LightClient from 'tendermint-light-client-js';

// Verify an untrusted block against a trusted one, given the specified options
// and current date/time.
let verdict = LightClient.verify(untrusted, trusted, options, now);

For an example of how to use this, please see the verifier-web example.